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/thumbimageupdater.cc

@@ -48,6 +48,8 @@ public:
 
 		Job():
 			tbe_(0),
+			priority_(NULL),
+			upgrade_(false),
 			listener_(0)
 		{}
 
@@ -86,7 +88,7 @@ public:
 	Glib::Cond inactive_;
 
 	void
-	processNextJob(void)
+	processNextJob()
 	{ 
 		Job j;