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/screen.h

@@ -118,6 +118,11 @@ class BasicScreen
 		/// cast to List if available or null pointer otherwise
 		///
 		virtual List *GetList() = 0;
+
+		/// When this is overwritten with a function returning true, the
+		/// screen will be used in tab switching.
+		///
+		virtual bool isTabbable() { return false; }
 		
 		/// Should be set to true each time screen needs resize
 		///