UI: Use Qt lib for screen info instead of x11 libs

All of the monitor discovery logic can be implemented with Qt. This
change removes all the x11 implementations in favor of Qt and removes a
fair amount of platform specific code.

UI/platform-osx.mm

@@ -36,17 +36,6 @@ bool GetDataFilePath(const char *data, string &output)
 	return !access(output.c_str(), R_OK);
 }
 
-void GetMonitors(vector<MonitorInfo> &monitors)
-{
-	monitors.clear();
-	for(NSScreen *screen : [NSScreen screens])
-	{
-		NSRect frame = [screen convertRectToBacking:[screen frame]];
-		monitors.emplace_back(frame.origin.x, frame.origin.y,
-				      frame.size.width, frame.size.height);
-	}
-}
-
 bool InitApplicationBundle()
 {
 #ifdef OBS_OSX_BUNDLE