Use contaoWidget instead of widget for Widget-Options

[cboelter]

As discussed a few weeks ago, here is the pull request to fix the conflict between this bundle and the symfony form bundle. The behaviour is illustrated in this issue: #5

[dmolineus]

LGTM.

I'll have to do several other tasks to get ready for the 2,0 release:

• Check requirements (Twig 3.0 support?)
• Update diagnostics.yml
• Fix frontend templates (containing backend JS at the moment)

[dmolineus]

Thanks @cboelter, got merged manually.