extensionDownloader: Compile gsettings schemas after install

After an extension is installed, run `glib-compile-schemas` on its
`schemas` directory, if it exists.

This should avoid any endianess-related issues for extensions when
running GNOME Shell on varying architectures.

Co-authored-by: Marco Trevisan (Treviño) <mail@3v1n0.net>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2638>