Updater API flexibility enhanced

The local repository can now be changed at any time :
 - constructor definition changed
 - new setter 'setLocalRepository(directory)'
Updater documentation improvements
Consistent and easy to use error/warning handling
Add Doxygen file for documentation generation
Metatype registered at compile time
Prevent download failure due to unremoved file
Simplify fixing operation states
Improve consistency of progression signals
Fix : Download wasn't stopped when a skip is useful, causing size to be negative
Fix : Synchronisation between current position and download position failed

downloadSeek was decremented while dataRequest data were not consumed