Check can_highlight in tui_check_and_display_highlight_if_needed

Check can_highlight in tui_check_and_display_highlight_if_needed

tui_check_and_display_highlight_if_needed currently checks the
window's type; but this can be replaced with a check of
"can_highlight", making it more polymorphically-correct.

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

	* tui/tui-wingeneral.c
	(tui_check_and_display_highlight_if_needed): Check can_highlight.