Refine tab context menu

Split some things out of !814 (merged) as it's relatively self-contained.

Remove some duplicate actions, adding shortcuts to the menu. Hide pin/unpin when disabled. Add mute/unmute since the mute button is inaccessible from keyboard. Although really now the context menu is also inaccessible, see !814 (comment 906953) Menu key does nothing, Shift+F10 opens primary menu instead.

Remaining questions:

1. Should pin and unpin still have different mnemonics?
2. Should there be a detach action? It's not very accessible on touch
3. Placement/wording of mute/unmute?

CC @bertob