added custom width property

[JakobFelixJulius]

Hello! I added a custom width property so users can select a maximum width of SpotMenu.

This is viable for smaller screens or crowded menu bars, as the default behavior of SpotMenu is to disappear if there isn't enough space.

The property includes as custom HoverTextField to allow a hovering tooltip similar to the buttons as well as a UserPreferences entry to allow for persistence.

See attached gif.

Let me know what you think!