Change TUI window commands to be case-sensitive

Change TUI window commands to be case-sensitive

The TUI window-related commands like "focus" are case insensitive.
This is not the norm in gdb, and I don't see a good reason to have it
here.  This patch changes the TUI to be case sensitive, like the rest
of gdb.

gdb/ChangeLog
2019-09-20  Tom Tromey  <tom@tromey.com>

	* NEWS: Mention case-sensitivity of TUI commands.
	* tui/tui-win.c (tui_set_focus_command): Now case-sensitive.
	(tui_set_win_height_command, parse_scrolling_args): Likewise.
	* tui/tui-layout.c (tui_layout_command): Now case-sensitive.