Update System! (finally)

With this PR we close development of a first version of the update system. 
The system is almost self contained: class UpdateSystem has most of the logic and controls the flow.

To test this you are gonna need to place a valid release.json file under assets/appdata/release.json. You could use for example this one: https://github.com/e-ucm/ead/blob/master/editor/core/src/test/resources/appdata/validrelease.json

You can modify the release.json's appVersion to see if the confirmation dialog to start the download appears or not. Any appVersion below 1.0 should make the confirmation dialog appear suggesting that a new version is available.