Introduce tui_win_info::update_tab_width

Introduce tui_win_info::update_tab_width

This introduces a new tui_win_info::update_tab_width method, and
changes the TUI to call it.  This fixes another spot that was checking
the window type.

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

	* tui/tui-win.c (tui_source_window_base::update_tab_width): New
	method.
	(update_tab_width): Call update_tab_width method.
	* tui/tui-data.h (struct tui_win_info)
	(struct tui_source_window_base) <update_tab_width>: New methods.