Introduce tui_gen_win_info::reset method

Introduce tui_gen_win_info::reset method

This introduces the tui_gen_win_info::reset method and changes various
places to use it.  This led to the realization that the can_highlight
member only needs to be set during construction, so this patch makes
that change as well.  Finally, init_and_make_win is drastically
simplified.

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

	* tui/tui-layout.c (make_command_window): Don't set
	can_highlight.
	(show_source_disasm_command): Call the reset method.
	(show_data): Don't set can_highlight.  Call the reset method.
	(tui_gen_win_info::reset): Rename from init_gen_win_info
	(init_and_make_win): Simplify.  Return tui_gen_win_info.
	(show_source_or_disasm_and_command): Call the reset method.
	* tui/tui-data.h (struct tui_gen_win_info) <reset>: New method.
	(struct tui_cmd_window): Set can_highlight.