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/help.h

@@ -34,6 +34,7 @@ class Help : public Screen<Scrollpad>
 		
 		virtual void EnterPressed() { }
 		virtual void SpacePressed() { }
+		virtual bool isTabbable() { return true; }
 		
 		virtual bool allowsSelection() { return false; }