Create subclasses for different window types

Create subclasses for different window types

This changes the TUI so that each different major window type has its
own subclass.

gdb/ChangeLog
2019-06-25  Tom Tromey  <tom@tromey.com>

	* tui/tui-data.h (struct tui_win_info): Make constructor
	protected.  Make destructor virtual.  Add initializers.
	(tui_source_window, tui_data_window, tui_cmd_window): New
	classes.
	* tui/tui-data.c (tui_win_info): Rename from init_win_info.  Now a
	constructor.  Add "type" parameter.
	(tui_source_window, tui_data_window, tui_cmd_window): New
	constructors.
	(tui_alloc_win_info): Instantiate the appropriate subclass.