Move contents of tui_show_frame_info to new method

Move contents of tui_show_frame_info to new method

This moves much of the body of tui_show_frame_info to a new method on
tui_source_window_base.  This removes a check for the type of a
window.

gdb/ChangeLog
2019-08-15  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (struct tui_source_window_base)
	<maybe_update>: Declare.
	* tui/tui-stack.c (tui_show_frame_info): Call maybe_update
	method.
	* tui/tui-source.h (struct tui_source_window) <maybe_update>:
	Declare.
	* tui/tui-source.c (tui_source_window::maybe_update): New method.
	* tui/tui-disasm.h (struct tui_disasm_window) <maybe_update>:
	Declare.
	* tui/tui-disasm.c (tui_disasm_window::maybe_update): New method.