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/playlist.cpp

@@ -97,6 +97,8 @@ void Playlist::SwitchTo()
 	if (hasToBeResized)
 		Resize();
 	
+	if (myScreen != this && myScreen->isTabbable())
+		myPrevScreen = myScreen;
 	myScreen = this;
 	Items->Window::Clear();
 	EnableHighlighting();