core: frontend: extensions manager tabs aren't centered

[ES-Alexander]

Current behaviour

The tabs are intended to be centred, but seem to be pushed over to the left by the settings cog button:

Not a functional problem, just looks weird.

Expected or desired behaviour

The tabs should be centred within the card, such that the edge of the underline of the selected tab is aligned with the centre gap between the installed extensions.

When the space runs out I think the tabs should be cut off on the right by the settings cog, and able to scroll out from underneath it (which will be extra relevant once there's an extra tab for Bazaar).

Prerequisites

• I have checked to make sure that a similar request has not already been filed or fixed.