Data management UI

Adding UI for managing data sets (game systems, catalogues etc).

Browsing Gallery/Appspot content and selecting which items to download,
and showing which are currently installed locally.

The backing code is supposed to be done in #25

Features:
- Show a list of repositories from a Catpkg Gallery (see #25)
- Elements (repositories) can be selected to be locally loaded (downloaded into the browser memory)
- Elements should show some details like name, date, version
- Elements should show a message/icon that an updated version is available (compared to locally downloaded)

A repository can have different states:
- listed as "available to load"
- listed as "loaded locally" and then:
- it can have an "update available"
- or it can be "up to date"