Remove gdk_threads* calls

And bump the GDK and GTK+ required version to 3.6.

https://bugzilla.gnome.org/show_bug.cgi?id=710821
Update TotemAspectFrame from totem

https://bugzilla.gnome.org/show_bug.cgi?id=710821
Use a layout manager instead of constraints

This reduces property changes for every little change in the stage's
properties.

https://bugzilla.gnome.org/show_bug.cgi?id=710821