Make Plugin download page presentable

The current plugin page is plagued by several bugs and is in no shape to presented to users:

[ ] Even when a repo is configured and can be reached, a text is shown that the repo isn't reachable
[ ] There's no progress indication that a download has happened
[ ] When a plugin has been downloaded, it doesn't appear on the left-hand-side until TV is restarted
[ ] The user isn't notified that a restart is necessary in order for the new plugin to be used

Kittyfisto / Tailviewer