Assets refactoring

Quite a little refactoring.

This PR refactors assets naming and hierarchy. Now, the assets class that deals with application's resources does not extend EngineAssets, but a new asbtract class.
This is the new Assets hierarchy:
Assets (abstract class)
  |__ EngineAssets (handles game's assets for the engine)
  |       |__ EditorAssets (handles game's assets for the editor)
  |__ ApplicationAssets (handles the editor's own assets, like the skin and i18n files, including the preferences).

Also: This PR fixes #127, fixes #111, fixes #166 and perhaps (not sure) fixes #179

I kindly request to have this pr revised and merged (if possible) first thing in the morning... before other PR are pushed on top and a lot of rebases are needed.