textview: add undo/redo support to GtkTextView

textview: add undo/redo support to GtkTextView

This builds upon the GtkTextHistory helper to provide undo and redo support
for the GtkTextView widget and GtkTextBuffer object.

You can undo/redo using familiar shortcuts such as Primary+Z,
Primary+Shift+Z, ad Primary+Y.

Developers that wish to disable undo, should set the
GtkTextBuffer:enable-undo property to FALSE.

You can wrap irreversible actions
gtk_text_buffer_begin_irreversible_action() and
gtk_text_buffer_end_irreversible_action(). This will cause the undo stack
to drop all undo/redo actions and the changes made between them will be
the "initial state" of the buffer.

Calling gtk_text_buffer_set_text() will do this automatically for you.