screen_switcher between current and last screen

Setting the new config variable "screen_switcher_browser_only" to "no"
will make the "key_screen_switcher" key switch between the current and
the last used screen, sort of like the Alt+Tab window switching
mechanism in many window managers.

This (intendedly) works only for the "main" screens (playlist, browser,
media library, etc.) and not for info/lyrics/server info. Those screens
can be closed with the screen_switcher key, but the will not be
remembered.

indentation correction in config

src/settings.h

@@ -187,6 +187,7 @@ struct ncmpcpp_config
 	bool header_text_scrolling;
 	bool statusbar_visibility;
 	bool centered_cursor;
+	bool screen_switcher_browser_only;
 	bool autocenter_mode;
 	bool wrapped_search;
 	bool space_selects;