Big refactor concering assets.
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.
Formatting and fixing travis
Merge pull request #190 from jtorrente/assets-refactoring-2

Big refactor concering assets.
AddScene Improvements: javadoc & code structure
Improvements in preferences' javadoc
Improvements in ChangeInitialScene: javadoc and input validation
Improvements in NewGame: input validation
Improvements in CompositeCommand javadoc
Minor fix in SidesLayout
Minor refactor in BaseGallery (@rotarudan and @gorco be aware of this): galleryTable => galleryGrid
Desktop platform: moving the construction of JavaRequestHelper to the DesktopPlatform constructor
Code legibility improvements in the view: mainBuilder and ScenesList

Javadoc + removal of anonymous classes
Fixes #111 and perhaps #179 (needs to be checked)
Formatting
Merge pull request #192 from jtorrente/minor-improvements-and-111-179-166

Minor improvements and 111 179 166
Typo