UI: Translate hotkey labels in configuration

Another request from GillianMC.

The translated strings have been placed in a separate "Hotkeys" context as an alternative
 to having to add the tr function to the Config class, or adding them to ConfigureHotkeys
context which is quite long. The English strings get attached to the items in the Action
column as "data", and are used for RetranslateUI and saving the hotkey configuration.