Create tui_disasm_window

Create tui_disasm_window

This introduces the new tui_disasm_window class, which represents a
disassembly window.  It shares a lot of behavior with the source
window, so a new tui_source_window_base class is also created.

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

	* tui/tui-data.h (struct tui_source_window_base): New struct.
	(struct tui_source_window): Derive from tui_source_window_base.
	(struct tui_disasm_window): New struct.
	* tui/tui-data.c (tui_source_window_base::clear_detail): Rename
	from tui_source_window::clear_detail.
	(tui_source_window_base): Rename from tui_source_window.
	(~tui_source_window_base): Rename from ~tui_source_window.
	(tui_alloc_win_info): Create a tui_disasm_window.