Big refactor concering assets.

This fixes #127

Now we have a superclass called Assets, abstract, which provides basic functionality for every asset manager in the application

Two classes extends Assets:
- GameAssets deals with assets of the game (for the engine): i18n, images, game.json, sceneXXX.json etc
- ApplicationAssets deals with assets of the application (the editor): skin, i18n, default preferences, etc.

We have also two classes extending GameAssets:
- MockGameAssets: for testing
- EditorGameAssets: extended functionality for game assets in the editor.

Also: now default preferences are loaded by ApplicationAssets

There are more than a couple of open issues related to assets: we should discuss & close em asap:
#189, #167, #121, #183