Closes#21920.
Weirdly enough this was semeingly fixed once before in ancient times in
3891f467a3, but then unfixed again in
566e09083f. The second change is no longer
needed since the toolbar became opaque in #9447.
Changes all usages of `KeyCombination.ReadableString()` to
`ReadableKeyCombinationProvider.GetReadableString()`.
Subscribing to `KeymapChanged` is only required in `KeyButton`.
All other places query `GetReadableString()` every time.