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/mydiagonalcurve.cc

@@ -806,6 +806,7 @@ void MyDiagonalCurve::updateBackgroundHistogram (LUTu & hist) {
         bghistvalid = false;
 
     mcih->pending++;
+    // Can be done outside of the GUI thread
     g_idle_add (diagonalmchistupdateUI, mcih);
 
 }