Add <project_license> to the AppData file
menu-spec: Fix translation of some categories

The msgctxt for the subcategories of 'Development Tools' was
wrong, causing the translations to not be picked up.

https://bugzilla.gnome.org/show_bug.cgi?id=729355
Updated Lithuanian translation
More menu spec translation context fixes

https://bugzilla.gnome.org/show_bug.cgi?id=729355
Depend on a new appstream-glib
Updated French translation
[l10n] Added Catalan (Valencian) translation
Update Catalan translation
Updated Spanish translation
trivial: Fix typo in code comment
Make the project licenses hyperlinked to the SPDX site
Never put 'Complicated' in the license field
Updated Polish translation
Updated Slovenian translation
Updated Greek translation
Updated Czech translation
shell updates: Remove a forgotten debug statement
shell updates: Remove NULL checks around g_list_free

NULL is a valid GList; no need to special case it.
shell updates: Instantiate a new update dialog each time
shell: Remove unneeded g_clear_object calls

We just created and 0-initialized the BackEntry; no need to clear the
members again.