Introduce tui_source_window_base::set_contents method

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.