Document TUI improvements in the manual & NEWS

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>
	    Hannes Domani  <ssbssa@yahoo.de>

	* NEWS: Add new "TUI Improvements" section and mention mouse
	support and that unrecognized special keys are now passed to
	GDB.  Mention Python Window.click in the Python improvements
	section.

gdb/doc/ChangeLog:
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>

	* gdb.texinfo (TUI): <TUI Mouse Support>: New node/section.

Co-Authored-By: Hannes Domani <ssbssa@yahoo.de>

Change-Id: I0d79a795d8ac561fd28cdc5184bff029ba28bc64