Provide functionality for add-on development

[cwisniew]

Identify the Bug or Feature request

fixes #3460

Description of the Change

This change adds the following features to the development tab of the dialog from File -> Add On Libraries.

• Directory to monitor for Add-on development
• Ability to import Add-ons within this directory
• Create the directory structure and example json files for new add on
• Create a .mtlib file based on the selected directory

Possible Drawbacks

This still requires the user to import the add-on from the directory by clicking a button; add-on importing for large libraries is not something that is suitable to be done automatically on every file change. This is especially the case for windows where the file system often reports multiple change events for a single change. But it is one step closer to providing an API that an IDE could use to trigger importing of an add-on.

Documentation Notes

This change adds the following features to the development tab of the dialog from File -> Add On Libraries.

• Directory to monitor for Add-on development
• Ability to import Add-ons within this directory
• Create the directory structure and example json files for new add on
• Create a .mtlib file based on the selected directory

Release Notes

• Provide functionality for add-on development

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This change is 

[thelsing]

Would it also be possible to autoinstall an Add-on from a github-repo? i.e. download the repo as zip, rename to mtlib and import?

[cwisniew]

Would it also be possible to autoinstall an Add-on from a github-repo? i.e. download the repo as zip, rename to mtlib and import?

This is the eventual goal (its why there is a git URL in the add on) but not as part of this change