UI: Don't include properties-view.hpp in header

Replace header include with forward to reduce needless header includes
and thus reduce compile time.

obs/window-basic-properties.cpp

@@ -20,6 +20,7 @@
 #include "window-basic-main.hpp"
 #include "qt-wrappers.hpp"
 #include "display-helpers.hpp"
+#include "properties-view.hpp"
 
 #include <QCloseEvent>
 #include <QScreen>

obs/window-basic-properties.hpp

@@ -22,8 +22,7 @@
 #include <memory>
 #include <obs.hpp>
 
-#include "properties-view.hpp"
-
+class OBSPropertiesView;
 class OBSBasic;
 
 #include "ui_OBSBasicProperties.h"