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/guiutils.cc

@@ -28,6 +28,11 @@
 
 using namespace std;
 
+#if TRACE_MYRWMUTEX==1 && !defined NDEBUG
+unsigned int MyReaderLock::readerLockCounter = 0;
+unsigned int MyWriterLock::writerLockCounter = 0;
+#endif
+
 Glib::ustring escapeHtmlChars(const Glib::ustring &src) {
 
     // Sources chars to be escaped