feature: disable postpone and skip buttons while shortcuts are disabled

[deltragon]

Description

Resolves #566.

This PR locks the Skip/Postpone button as well while the keyboard shortcuts are disabled.
It also greys out the buttons, and removes the hover style to signal that they are non-interactive.

[deltragon]

I've opened this as a draft because I am unsure about this.
I do think I slightly prefer this over the limitconsecutiveskipping plugin, personally.
However, as this is implemented right now, it will be enabled by default - together with limitconsecutiveskipping. That may be a bit much.
So maybe this should controlled by its own setting, separate from the keyboard shortcuts, and off by default.