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>