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

gtk/gtkentry.h

@@ -306,6 +306,10 @@ gint         gtk_entry_get_current_icon_drag_source      (GtkEntry             *
 GdkWindow  * gtk_entry_get_icon_window                   (GtkEntry             *entry,
                                                           GtkEntryIconPosition  icon_pos);
 
+gboolean    gtk_entry_im_context_filter_keypress         (GtkEntry             *entry,
+                                                          GdkEventKey          *key);
+void        gtk_entry_reset_im_context                   (GtkEntry             *entry);
+
 
 /* Deprecated compatibility functions
  */