Mediabrowser

The Kickstarter is extended to generate a basic media browser, that lets editors upload generic files like images and PDFs and allows to choose one or more of these assets for linklist and the upcoming reference or referencelist attribute.

The media browser should be triggered in the inplace GUI when editing a reference or referencelist attribute. This functionality needs to be implemented in the related issue.

The browser opens in a modal dialog comparable to the widget or class browser. All styles are part of the application and do not rely on any CSS framework like Twitter Bootstrap.

Fixes #116.