Added on back pressed default behavior.
Added exit dialog for the InitialScreen.
Fixed a bug that would cause the galleries not to get updated in some cases.
Merge pull request #195 from RotaruDan/AndroidBackButtonFucntionality

Android back button functionality.
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.
Added search-index support for new models (instead of only-updates)
Moved test-i18n messages to separate folder;
fixed i18n test to match
Added a dependency to engine-desktop test-classes for editor testing
Moved teststyle into the hosting game, testgame.
Partially addresses #186
Incremental search, with tests.