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/global.h

@@ -28,7 +28,9 @@
 namespace Global
 {
 	extern BasicScreen *myScreen;
-	extern BasicScreen *myOldScreen;
+	extern BasicScreen *myOldScreen;	// for info, lyrics, popups
+	extern BasicScreen *myPrevScreen;	// "real" screen switching
+						// 	(browser, search, etc.)
 	
 	extern Window *wHeader;
 	extern Window *wFooter;