Use GtkIMContext instead of ClutterIMContext

Use GtkIMContext instead of ClutterIMContext

Add StIMText, which is a drop-in replacement for ClutterIMText but
uses GtkIMContext instead of ClutterIMContext.

StIMText doesn't have preedit support (would need ClutterText
changes), so isn't going to be useful for complicated input methods,
but is good enough to get dead keys and similar working.

entry.js: Simple test case of StEntry
gnome-shell.modules: Remove clutter-imcontext module

https://bugzilla.gnome.org/show_bug.cgi?id=597471