Big changes here in CameraGUI:

-Load camera icon view thumbnail on the fly. Do not load all of then at the first time, which time consuming.

-Reduce memory allocation of thumbs to only visible items.

-Reduce time loading at camera connection to only parse file system information not metadata from files, which time consuming.
Definitivly, camera connection duration is reduced by 10~20.

-Camera icon view are sorted by date from file system. Metadata are loaded on the fly, at the same time than thumbnails,
especially to display photo properties from tool-tips and right sidebar.
Note : at this commit, only USM camera is patched, GPhoto driver still todo.

-In camera drivers, simplify api and add new virtual methods to get full metadata container (not only exif). Always get metadata
through separate thread to reduce time latency in gui.

-In camera sidebar implementation, remove all dependencies to CameraIconView and CameraIconItem for future Qt4 model/view port
of camera icon view.

-Bug fix in UMSCamera driver about to load date time-stamp from file system : information is never stored through imagescanner.

-General implementation polish and optimizations, especially in camera item info container.