* Undo support:
 - finalized the history (undo) panel with global undo / redo operations
(Ctrl+Z / Ctrl+Y)
 - added undo support to almost all ROI actions from GUI.
 - modified mergeable property so we don't have anymore non significant
undoable edit (easier to manage)
* Loader:
 - improved image preview display when multiple importer are available
 - Loader.loadThumnail(..) now search in all compatible importer
* Loci importer: better retrieving of channel color from metadata
* added the ugly PluginNoEDTConstructor interface to avoid plugin
creation in the swing graphics thread (needed for retro compatibility).
Be careful as some Plugin abstract classes now use this interface.
* disable user link action as others features are not ready yet.
* IcyBufferedImageUtil.convertToType(..) now uses safe type conversion
as we always prefer it (even if it's bit slower).
* added ImageUtil.isGray(image) method to know if the specified RGB
image only contains gray pixels.
* minor fix with internet monitor.
* pre load the importers classes to avoid non responsive UI for some
seconds on first open operation.
* added MetaDataUtil.getChannelColor(..) method
* ROI2DShape: added addNewPoint(..) and removePoint(..) method