File chooser fixes
Move away from GtkFileChooserDialog and use GtkFileChooserNative instead, creating it on demand instead of always keeping an instance around. This also adds "image/*" mime-type filter where it was missing and cleans up some unused file chooser-related stuff.
Fixes #581 (closed)
Related to Librem5/debs/gtk#33 (should at least make Chatty not trigger it anymore)