remove canvastest
* added LOCI metakit library (needed for some image format).
* added Overlay class to replace the Painter interface:
 - add support for overlay priority.
 - automatic change event dispatch to sequence object.
 - many classes has been modified internally to use the new Overlay
class, backward compatibility should be provided.
 - many others improvements, don't use the painter anymore.
* reworked Layer class:
 - handle new Overlay class.
 - dispatch event only for Layer properties change.
 - added fixed (cannot be removed) and readOnly (cannot change
name) properties.
* we can now disable the automatic image ordering when loading sequence
from several files.
* results from the search bar are now sorted depending their relevance.
* changed class loader search order in plugin class loader (system
first --> local last).
* several tweaks and fixes.