Extension library items should be accessible with tab key

[towerofnix]

Expected Behavior

Pressing tab to navigate the extension library should work as it does with the sprite library.

Actual Behavior

Tabbing only moves through the filter input and close (X) button.

Steps to Reproduce

Open the extension library (by clicking on the "Extensions..." button), then press tab a few times.

Operating System and Browser

Debian (testing), Firefox 58.

[mrjacobbloom]

This is a specific case of #907, which was mostly resolved by #986 but I guess didn't cover the extensions library