Added api to reset the im context in GtkTextView and GtkEntry

Added api to reset the im context in GtkTextView and GtkEntry

Also, added api to allow an input method to internally handle
key press and release events in the GtkTextView and GtkEntry
cases.
This is simply a wrapper to the gtk_im_context_filter_keypress()
function, but It's added to not access the ->im_context
directly.
Based on a Christian Dywan patch

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=163251