JBIDE-18670 HTML5 Palette: Palette model refactoring

new palette model implemented
Version with index 0 is the earliest version. Before, by default the latest version was taken.
I suggest either return group.getVersions()[group.getVersions().length - 1];
or method getLatestVersion() can be added to IPaletteGroup (then it is better to return IPaletteVersionGroup than String).