Once installed, the "Install" button is replaced with Settings, Disable, and Remove buttons.
The lack of real estate is definitely an issue here. I had to remove the category in order to make room for the buttons. That could be alleviated by using icons for the buttons instead of textual buttons, but that could make them more obscure, and also seems inconsistent with the fact that the "Install" button was textual in the original view.
Also, there wasn't a natural place to put a more visual indication that these extensions are installed. In later screenshots I cheat and replace the icon area with a checkmark.