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());
+}