UI: Add function to get main window more easily

Adds OBSBasic::Get() to just quickly return the main window.

UI/window-basic-main.cpp

@@ -6857,3 +6857,8 @@ void OBSBasic::ResizeOutputSizeOfSource()
 	ResetVideo();
 	on_actionFitToScreen_triggered();
 }
+
+OBSBasic *OBSBasic::Get()
+{
+	return reinterpret_cast<OBSBasic*>(App()->GetMainWindow());
+}

UI/window-basic-main.hpp

@@ -617,6 +617,8 @@ public:
 	void CreatePropertiesWindow(obs_source_t *source);
 	void CreateFiltersWindow(obs_source_t *source);
 
+	static OBSBasic *Get();
+
 protected:
 	virtual void closeEvent(QCloseEvent *event) override;
 	virtual void changeEvent(QEvent *event) override;