Extension manager takes long time to manage local extensions

[mubaidr]

Behavior: Opening extension manager, new list of extensions is fetched each time.

Issue: If i only want to remove or view existing extensions i had to wait until extension list is fetched from Internet.

Expected: Installed extensions are available for management as soon as extension manager is opened. New extension list should be fetched async in the background or only when user switch to on-line tab.

[peterflynn]

@mubaidr It is already fetched async in the background. But there is a bug where we jump back to the Available tab when it's done loading. Often this happens so quickly that it looks like clicking the Installed tab didn't even work. We should fix that.

It also looks like rendering the list once it's been fetched is slow enough to cause a noticeable freeze (feels like 3/4 sec or so on my Win 7 machine). We should consider rendering the list in chunks to avoid blocking the UI -- adding 'performance' tag for that.

[dangoor]

Reviewed medium priority in the extension manager.