SpaceLobby: enable showing the `buttons_view` on mobile without mouse hover

Currently the buttons are only shown upon hover, which means they can never be shown on a mobile device. This should be easy to fix, likely by maintaining a "hovered" or "down" state that toggles any time the user taps on a SubspaceEntry. So the user should be able to tap on an entry to show the buttons, and then tap again to hide the buttons.