Separate out execution-info window

Separate out execution-info window

This pulls the EXEC_INFO_WIN case out into its own subclass of
tui_gen_win_info.  This lets us remove an element from
union tui_which_element.

gdb/ChangeLog
2019-06-25  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.c
	(tui_exec_info_window::maybe_allocate_content): New method.
	(tui_set_exec_info_content, tui_show_exec_info_content): Update.
	* tui/tui-layout.c (init_and_make_win): Add EXEC_INFO_WIN case.
	(make_source_or_disasm_window): Add cast.
	* tui/tui-data.h (union tui_which_element) <simple_string>:
	Remove.
	(struct tui_source_info): New.
	(struct tui_source_window_base) <execution_info>: Change type.
	* tui/tui-data.c (init_content_element): Remove EXEC_INFO_WIN
	case, and add assert.
	(tui_alloc_content): Add assert.