Move some functions into Utils::OS namespace

Also remove `Utils::Misc::getUserIDString()` since there are no use of
it.

src/gui/macosdockbadge/badgeview.mm

@@ -28,6 +28,9 @@
  */
 
 #import "badgeview.h"
+
+#include <QString>
+
 #include "base/utils/misc.h"
 
 static const CGFloat kBetweenPadding = 2.0;