Break out the sources dialog as a separate object

At the same time, move to a separate GtkBuilder template.

https://bugzilla.gnome.org/show_bug.cgi?id=726413
sources dialog: Set headerbar as an internal child in the .ui file

https://bugzilla.gnome.org/show_bug.cgi?id=726413
sources dialog: Fade in the loading spinner

This avoids briefly flickering the spinner when the loading time is fast
enough (which is the case with newer PK backends like Hawkey).

https://bugzilla.gnome.org/show_bug.cgi?id=726413
build: Use GNOME_COMPILE_WARNINGS
updates: Add more strings to the last checked string

These were left out of 3.12 at the request of the i18n team.

https://mail.gnome.org/archives/gnome-i18n/2014-March/msg00042.html
Port to using Appstream-Glib in the AppStream and AppData plugins

This adds a new hard dep, but removes a ton of code. You can find information
about this lib here: http://people.freedesktop.org/~hughsient/appstream-glib/