Don't call gdk_threads_init()

Accessing gdk from outside the main thread is deprecated.

This also replaces an occurence of gdk_threads_add_idle_full() with an
idle source that is dispatched to the main thread.

https://bugzilla.gnome.org/show_bug.cgi?id=721755
Prepare for release 3.11.4
Post-release version bump
Finnish translation update
Updated Czech translation
[l10n] Updated Estonian translation
Updated Greek translation