WIP: Install themes to $prefix/share/themes

Installs themes (those from the themes submodule) to $prefix/share/themes instead of $prefix/share/inkscape/themes. Defines $XDG_DATA_DIRS for AppImage in the AppRun launcher script. Our Windows bundle doesn't need $XDG_DATA_DIRS, and the macOS bundle already sets $XDG_DATA_DIRS. System-wide installations to the default /usr prefix also won't need $XDG_DATA_DIRS.

Tested with:

• macOS artifacts from CI
• AppImage artifacts from CI
• local Windows build (CI currently broken)

For installations into non-standard locations you need to set XDG_DATA_DIRS yourself, to a value like XDG_DATA_DIRS=/yourcustomprefix/share/:/usr/local/share/:/usr/share/.