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/previewhandler.cc

@@ -23,8 +23,8 @@
 using namespace rtengine;
 using namespace rtengine::procparams;
 
-PreviewHandler::PreviewHandler () : image(NULL) {
-    
+PreviewHandler::PreviewHandler () : image(NULL), previewScale(1.) {
+
     pih = new PreviewHandlerIdleHelper;
     pih->phandler = this;
     pih->destroyed = false;