Show version installed in the details

So it serves as a notifier that the plugin has been successfully
installed.

We could probably have a message box or a status bar message too.
The install plugin button is selectively enabled

...for plugins that are not already installed.

For the plugins that are installed, maybe we could turn the button to
'Update plugin', so it updates the plugin (if an update is available)
Added a 'Hide installed plugins' checkbox to the downloads page

We could have a setting that defines the default state of the checkbox.

The checkbox feels a bit oddly placed, a push button would've been
better?