Add package.xml FreeCAD package metadata to support addon manager, various fixes

[mroote]

Adds a package.xml file to support installation directly through the FreeCAD Addon Manager.

Note The cadquery package will also have to be added in the FreeCAD-addons allowed dependencies file.

Also includes various small fixes:

• Fixed when user does not have a home dir env var available (Windows)
• Close text editor window when switching away from workbench
• Fixed #3