Add destination path preview label to batch queue panel.

Label initially says "Destination path for the first selected image appears here"
so the feature is discoverable. Select a file and it shows the destination path
that would be used if it is the first file to be processed and the file does
not already exist. Label is updated as the template is edited, so you can see the
effect of the edits. If "Save to folder" is selected, the previewed path uses
that folder.

rtgui/batchqueuepanel.h

@@ -42,6 +42,7 @@ class BatchQueuePanel : public Gtk::Box,
     Gtk::CheckButton* qAutoStart;
 
     Gtk::Entry* outdirTemplate;
+    Gtk::Label* destinationPreviewLabel;
     MyFileChooserButton* outdirFolder;
     Gtk::Button* outdirFolderButton;
     Gtk::RadioButton* useTemplate;
@@ -72,6 +73,7 @@ public:
     // batchqueuelistener interface
     void queueSizeChanged(int qsize, bool queueRunning, bool queueError, const Glib::ustring& queueErrorMessage) override;
     bool canStartNext() override;
+    void setDestinationPreviewText(const Glib::ustring& destinationPath) override;
 
 private:
     void startBatchProc ();