Use the header divider for the plugin view tabbar

The old tab bar wasted space and didn't look very good under the
header's divider, so this merges the tab bar with the header, like the
Pulse/Overview tabs in the phone layout.

- Get rid of the old tab bar
- Display the tabs in the header's divider
- Animation the transitions between plugin items
Fix many of the QML errors and warnings

- Check that Loader's item is valid
- Only load reviews if the AppId is valid
- Check the actual properties in GitHub
Connect available assignees to GitHub API