core: Nullability fixes

core: Nullability fixes

Almost all of these are just the necessary ‘(!)’ annotations to allow the
nullability check to pass. There were few, if any, actual bugs found by the
check (which either means folks is perfect, or Vala's nullability checking
is imperfect).

This brings us down from 296 nullability errors to just below 50.

The work was done by compiling folks with valac’s
--enable-experimental-non-null flag. We’re not ready to add the flag to
VALAFLAGS permanently yet, since this commit depends on various annotation
fixes in GLib (and similarly, the next one depends on several in EDS).
However, the fixes themselves should be valid without the flag.

This depends on (at least):
 • https://bugzilla.gnome.org/show_bug.cgi?id=666700
 • https://bugzilla.gnome.org/show_bug.cgi?id=666699