Change how TUI windows are instantiated

Change how TUI windows are instantiated

This adds a new global that maps from window names to window
constructor functions, and then changes tui_get_window_by_name and
validate_window_name to use it.  This is another step toward
user-defined window types.

gdb/ChangeLog
2020-02-22  Tom Tromey  <tom@tromey.com>

	* tui/tui-layout.c (make_standard_window, get_locator_window): New
	functions.
	(known_window_types): New global.
	(tui_get_window_by_name): Reimplement.
	(initialize_known_windows): New function.
	(validate_window_name): Rewrite.
	(_initialize_tui_layout): Call initialize_known_windows.

Change-Id: I9037aac550299b9d945899220a30c2d3af9dd0de