First try at adding expandable queue template help, using a Gtx::Paned.

Hiding doesn't work right yet. Click a thumbnail and the text disappears,
but the border between widgets doesn't move to the left side.
I tried using a Gtk::Expander, but couldn't get it to expand horizontally.

rtgui/batchqueuepanel.h

@@ -52,8 +52,10 @@ class BatchQueuePanel : public Gtk::Box,
 
     RTWindow* parent;
     BatchQueue* batchQueue;
+    Gtk::TextView* templateHelpTextView;
     Gtk::Box* bottomBox;
     Gtk::Box* topBox;
+    Gtk::Paned* middleSplitPane;
 
     std::atomic<bool> queueShouldRun;