Partially solving issue 1788: "Crashing during processing of queue"

It also correct a bug of wrong image orientation (was wrong in the File Browser while good in the Editor tab)

This patch also introduce several speed optimizations like:
   - switch between File Browser and Editor tab in Single Editor mode
   - asynchronous computation of the Batch Queue's thumbnails
   - quick start of RT even with a huge queue
   - less GUI overhead when applying a profile to multiple thumbs in the File Browser

rtgui/dirselectionlistener.h

@@ -24,6 +24,7 @@
 class DirSelectionListener {
 
   public:
+    virtual ~DirSelectionListener () {}
     virtual void dirSelected (const Glib::ustring& dirname, const Glib::ustring& openfile="") {}
 };