menuconfig: Extend dialog_textbox so that it can return to a scrolled position

We can now display other UI elements (menus) "on top" of a textbox and then
seemingly come back to it in the same state it was left.

Signed-off-by: Benjamin Poirier <bpoirier@suse.de>
Signed-off-by: Michal Marek <mmarek@suse.cz>