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.