TextInput event optimization

Consistency: rename PreviewTextInput event to TextInput
* Xwt has no preview events. To stop an event, the Handled property of the event arguments object should be used.

GTK: adds TextInput event support
* different implementation for Editable and non-Editable widgets
* Gtk.Editable (like Entry, TextView, etc.): simply handle TextInserted event
* Other widgets: use Gtk.IMMulticontext to get text input directly from the system IM