Move content_in_use to tui_source_window class

Move content_in_use to tui_source_window class

From scanning the source now, it's clear that the content_in_use field
is only used for the source window.  This patch moves the field there,
and changes it to be a bool at the same time.  (A future patch will
clean this up further, removing the field entirely.)

gdb/ChangeLog
2019-07-17  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.c (tui_clear_source_content)
	(tui_show_source_content, tui_show_exec_info_content)
	(tui_clear_exec_info_content): Update.
	* tui/tui-stack.c (tui_show_locator_content): Update.
	(tui_show_frame_info): Update.
	* tui/tui-source.h (tui_source_window): Don't declare.
	* tui/tui-source.c (tui_source_window::showing_source_p): Rename
	from tui_source_is_displayed.
	* tui/tui-data.h (struct tui_gen_win_info) <content_in_use>:
	Remove field.
	(struct tui_source_window_base) <content_in_use>: New field.  Now
	bool.
	(struct tui_source_window) <showing_source_p>: New method.
	(TUI_SRC_WIN): Change cast.
	* tui/tui-data.c (tui_initialize_static_data): Update.