We need to figure out what to do in the registry tab for items that are already installed. Right now we show a disabled, long "Installed" button and an enabled, short "Remove" button, which looks wrong. I'm not sure what the right thing to do is, though:

1. just have the Remove button (but that might not be an obvious enough cue that the extension is installed)
2. have the Remove button and have some other visual cue for installed extensions in the registry tab
3. just have the "Installed" indicator and don't let them remove the extension from this tab

I think I would prefer (2), but check with @larz0.