gdb/tui: Add command completion to winheight command.

gdb/tui: Add command completion to winheight command.

Share the window name completion code from the focus command with the
winheight command, providing window name completion for the winheight
command.

gdb/ChangeLog:

	* tui/tui-win.c (window_name_completer): New function.
	(focus_completer): Call window_name_completer.  All old content
	moved into window_name_completer.
	(winheight_completer): New function.
	(_initialize_tui_win): Rename variable.  Add completer to
	winheight command.  Update doc string on winheight.