Change tui_update_source_window to be a method
    
    This changes tui_update_source_window to be a method on
    tui_source_window_base.
    
    gdb/ChangeLog
    2019-08-16  Tom Tromey  <tom@tromey.com>
    
    	* tui/tui-winsource.h (struct tui_source_window_base)
    	<update_source_window>: Declare.
    	(tui_update_source_window): Don't declare.
    	* tui/tui-winsource.c
    	(tui_source_window_base::update_source_window): Rename from
    	tui_update_source_window.
    	(tui_source_window_base::rerender): Update.
    	* tui/tui-source.c (tui_source_window::maybe_update): Update.
    	* tui/tui-disasm.c (tui_show_disassem)
    	(tui_show_disassem_and_update_source)
    	(tui_disasm_window::maybe_update): Update.