Bump GTK+ version to only 2.18 for now

We might to push it farther in this release cycle, but we only need
2.18 for the GtkInfoBar/document-messages stuff in 8ea54993c5d599f82b6060550f304e06302b46d6.

@eht16 and others, is there other places to update? I suppose the Win32 build needs to be pushed past or to 2.18, etc.

I just did a search replace mostly from 2.16 to 2.18, we should review it closer.