gdb: Convert TUI windows names to lower case.

gdb: Convert TUI windows names to lower case.

This commit converts the window names for the TUI windows to lower case.
The windows names are already lower case in the documentation, and are
shown as lower case in some of the command completion results.

Given that all the commands that take a window name currently accept
upper or lower case, this commit just changes the window names to lower
case in the remaining places they are displayed by gdb.

gdb/ChangeLog:

	* tui/tui-data.h (SRC_NAME): Convert to lower case.
	(CMD_NAME): Likewise.
	(DATA_NAME): Likewise.
	(DISASSEM_NAME): Likewise.
	* tui/tui-win.c (tui_set_focus): Window names are now lower case.
	(tui_set_win_height): Likewise.
	(parse_scrolling_args): Likewise.