Add a small interface into Qt widgets to our own python wrapping

* This by no means replaces PySide2, but it allows python extensions to write
  simple UIs without needing to rely on PySide2, which might not be available
  (generally all windows builds have it as well as recent binary linux builds,
  but local windows builds may not and most linux builds probably won't).

qrenderdoc/Windows/PythonShell.h

@@ -47,6 +47,9 @@ public:
 
   ~PythonShell();
 
+  // for UI-forwarding helper classes
+  PythonContext *GetScriptContext();
+
   // IPythonShell
   QWidget *Widget() override { return this; }
 private slots: