File browser filter: Exposure Compensation (see issue 1003)

In order to use this new filter with previously browsed thumbnails there are 2 options:
Option 1. Global fix: clear cache/data folder

Option 2. Local Fix: select thumbs, right click, Cache /Clear from cache - partial
   reload folder in file browser

rtgui/cacheimagedata.h

@@ -54,6 +54,7 @@ class CacheImageData {
         Glib::ustring lens;
         Glib::ustring camera;
         Glib::ustring filetype;
+        Glib::ustring expcomp;
 
         // additional info on raw images
         int   rotate;