Improve debugging feedback for plugin developpers

[pmiossec]

by:

• logging configuration values during build
• fixing "AppVeyor" typo
• providing a link to msbuild target file as documentation on msbuild properties
• reordering properties in a slightly more logical order ('Source' first)

After:

[RussKie]

"For overwrite and valid values" this bit is confusing... Could you please help me understand what this means?

[pmiossec]

The plugin developer could overwrite the values of these properties to target a specific version like it is done here:
https://github.com/gitextensions/GitExtensions.GerritPlugin/blob/master/src/GitExtensions.GerritPlugin/GitExtensions.GerritPlugin.csproj.user

For example to start migrating to a version not yet released (that's what I did to migrate the plugin I maintain).

The goal of this change is:

1. don't loose time trying to find against which version of GitExtensions we are building
2. easily know where is located the version downloaded (because from what I get, you have to delete the folder manually if you want to use another version)
3. add this line of documentation to make easier for plugin developer how to change/overwrite the values and which one are valid

[RussKie]

Suggested change

	<Target Name="PreBuild" BeforeTargets="$(BuildDependsOn)">
	<Message Text="Executing prebuild event provided by GitExtensions.Extensibility" />
	<Message Text="Executing prebuild event provided by GitExtensions.Extensibility" Importance="high" />
	<Message Text="[GitExtensions.Extensibility] Source: $(GitExtensionsReferenceSource)" Importance="high" />
	<Message Text="[GitExtensions.Extensibility] Version: $(GitExtensionsReferenceVersion)" Importance="high" />
	<Message Text="[GitExtensions.Extensibility] Download Path: $(GitExtensionsDownloadPath)" Importance="high" />
	<Message Text="[GitExtensions.Extensibility] GitExtensions binaries path: $(GitExtensionsPath)" Importance="high" />
	<Message Text="[GitExtensions.Extensibility Documentation] For overwrite and valid values, see: https://github.com/gitextensions/gitextensions.extensibility/blob/master/src/GitExtensions.Extensibility/build/GitExtensions.Extensibility.targets" Importance="high" />
	<MakeDir Directories="$(_GitExtensionsDownloadPath)" />
	<Error Condition="!Exists($(GitExtensionsExecutablePath)) and !Exists($(_GitExtensionsDownloadScriptPath))" Text="Path to Git Extensions portable download script is wrong. Current value '$(_GitExtensionsDownloadScriptPath)'." />
	<Exec Condition="!Exists($(GitExtensionsExecutablePath))" Command="powershell.exe -NoProfile -ExecutionPolicy Unrestricted $(_GitExtensionsDownloadScriptPath) -ExtractRootPath $(_GitExtensionsDownloadPath) -Version $(GitExtensionsReferenceVersion) -Source $(GitExtensionsReferenceSource)" />
	<Message Text="Completed prebuild event provided by GitExtensions.Extensibility" />
	<Message Text="Completed prebuild event provided by GitExtensions.Extensibility" Importance="high" />
	<Target Name="PreBuild" BeforeTargets="$(BuildDependsOn)">
	<Message Text="[GitExtensions.Extensibility] Refer to documentation for list of variables and supported values at https://github.com/gitextensions/gitextensions.extensibility/blob/master/src/GitExtensions.Extensibility/build/GitExtensions.Extensibility.targets" Importance="high" />
	<Message Text="[GitExtensions.Extensibility] Executing prebuild event..." Importance="high" />
	<Message Text="[GitExtensions.Extensibility] Source: $(GitExtensionsReferenceSource)" Importance="high" />
	<Message Text="[GitExtensions.Extensibility] Version: $(GitExtensionsReferenceVersion)" Importance="high" />
	<Message Text="[GitExtensions.Extensibility] Download path: $(GitExtensionsDownloadPath)" Importance="high" />
	<Message Text="[GitExtensions.Extensibility] Binaries path: $(GitExtensionsPath)" Importance="high" />
	<MakeDir Directories="$(_GitExtensionsDownloadPath)" />
	<Error Condition="!Exists($(GitExtensionsExecutablePath)) and !Exists($(_GitExtensionsDownloadScriptPath))" Text="Path to Git Extensions portable download script is wrong. Current value '$(_GitExtensionsDownloadScriptPath)'." />
	<Exec Condition="!Exists($(GitExtensionsExecutablePath))" Command="powershell.exe -NoProfile -ExecutionPolicy Unrestricted $(_GitExtensionsDownloadScriptPath) -ExtractRootPath $(_GitExtensionsDownloadPath) -Version $(GitExtensionsReferenceVersion) -Source $(GitExtensionsReferenceSource)" />
	<Message Text="[GitExtensions.Extensibility] ...done!" Importance="high" />