Allow users to choose between fixed input sources list and MRU input sources list

For users who have more than 2 input sources (English, Chinese and Japanese for me), the MRU input sources list is totally a mental burden, because I don't know what the next input source is, and I have to wait for the popup to decide how much key press I need. Also, we allow user to tweak their own input sources sequence in Control Center, but we don't use it in GNOME Shell.

This MR adds support for fixed input sources list. Fixed or MRU list could be toggled by use-mru-sources key of org.gnome.desktop.input-sources schema, so we could add an option for it in Control Center, like what we have for per-window/global input sources.

Fixes #5687.

This depends on gsettings-desktop-schemas!64.