Add optional full_window argument to TuiWindow.write

Add optional full_window argument to TuiWindow.write

To prevent flickering when first calling erase, then write, this new
argument indicates that the passed string contains the full contents of
the window.  This fills every unused cell of the window with a space, so
it's not necessary to call erase beforehand.

gdb/ChangeLog:

2021-05-27  Hannes Domani  <ssbssa@yahoo.de>

	* python/py-tui.c (tui_py_window::output): Add full_window
	argument.
	(gdbpy_tui_write): Parse "full_window" argument.

gdb/doc/ChangeLog:

2021-05-27  Hannes Domani  <ssbssa@yahoo.de>

	* python.texi (TUI Windows In Python): Document "full_window"
	argument.