renderer: Split mutable geometry and place it directly in GtkKeyboard

Geometry is now permanently married to the widget rather the renderer. While geometry is not always defined, C doesn't support sum types, so checks won't be enforced by the compiler. It's OK to pretend there's always some geometry to avoid crashes.

eek/eek-gtk-keyboard.h

@@ -28,6 +28,7 @@
 #include <glib.h>
 #include <gtk/gtk.h>
 
+#include "eek/eek-renderer.h"
 #include "eek/eek-types.h"
 
 struct submission;