Add Modifiable Keyboard Shortcut for Mask Visibility

Whilst the mask visibility options have preset keyboard shortcuts (M and ALT+M on Windows), unlike almost all other functions, these cannot be modified in Edit Keyboard Shortcuts. I would like to see this corrected. I would then hope be able to use one shortcut key to toggle between mask visibility "on", "greyscale", and "off". This minor change would provide additional customisation and therefore a slightly better user interface.