Introduce tui_source_window_base::set_contents method
    
    This introduces the tui_source_window_base::set_contents method and
    implements it in the subclasses.  This removes a check of the window
    type.
    
    gdb/ChangeLog
    2019-08-16  Tom Tromey  <tom@tromey.com>
    
    	* tui/tui-winsource.h (struct tui_source_window_base)
    	<set_contents>: Declare.
    	* tui/tui-winsource.c
    	(tui_source_window_base::update_source_window_as_is): Update.
    	* tui/tui-source.h (struct tui_source_window) <set_contents>:
    	Declare.
    	(tui_set_source_content): Don't declare.
    	* tui/tui-source.c (tui_source_window::set_contents): Rename from
    	tui_set_source_content.
    	* tui/tui-disasm.h (struct tui_disasm_window) <set_contents>:
    	Declare.
    	(tui_set_disassem_content): Don't declare.
    	* tui/tui-disasm.c (tui_disasm_window::set_contents): Rename from
    	tui_set_disassem_content.