Tree browser home directory: user home or project base directory

[torsten-rupp]

Sometimes it is more convenient if the home button in the tree browser go to the project base directory instead of the user home directory.

The pull request

#1095

add a combo box to the tree plugin preferences where the prefered home directory (user home or project base) can be selected. The default is the existing behaviour where "home" to go the user home directory. If the combo box is set to "project base" then "home" go to the project base directory.