Fix GtkEntry drawing to provide visual feedback when editable=FALSE.

Fix GtkEntry drawing to provide visual feedback when editable=FALSE.

GtkEntry currently draws exactly the same no matter what the state of the
'editable' property. This is pretty confusing for users because there's
no visual feedback at all, it just seems like their keyboard is broken.

This change adds a "read-only" class to the StyleContext, which will
continue to allow the user to select/copy the text, but will draw the
entry as if it were insensitive, providing some indication that the
contents can't be changed.

Signed-off-by: David Trowbridge <trowbrds@gmail.com>
http://bugzilla.gnome.org/show_bug.cgi?id=694831