UI: Move properties window creation for new sources

Currently creating new sources can cause a deadlock:
OBSBasicSourceSelect locks the scene mutex when adding a new source
(required to add invisible sources), and later OBSBasic tries to
lock the graphics mutex (via CreatePropertiesWindow); meanwhile the
graphics thread is holding the graphics mutex and tries to lock each
scene as it renders them, resulting in a (non-obvious from the code)
lock ordering conflict.

Moving the CreatePropertiesWindow call out of the locked scene mutex
restores the previous lock ordering; in addition, the requirement
for keeping sourceSceneRefs for opening that initial properties
window is removed

obs/window-basic-source-select.hpp

@@ -45,4 +45,6 @@ private slots:
 
 public:
 	OBSBasicSourceSelect(OBSBasic *parent, const char *id);
+
+	OBSSource newSource;
 };