move searching to Menu class, also remember last constraint

this improvement also make that list of found items is not cleared
if one switches to another screen. it's remebered until the content
of screen changes (e.g. item is deleted from playlist)

src/clock.cpp

@@ -83,7 +83,6 @@ void Clock::SwitchTo()
 	if (hasToBeResized)
 		Resize();
 	
-	CLEAR_FIND_HISTORY;
 	myScreen = this;
 	myPlaylist->Main()->Hide();
 	redraw_header = 1;