Aurelia: Startup dim level, -1 is passed as unsigned type

[lgorter-victron]

In the Aurelia configuration, when changing the "startup dim level" setting to "Restore dim level from memory" (value -1), the new value is passed as an unsigned 32-bit integer.
I added a work around in vedirect-interface, by explicitely casting the value to a signed type. This should be fixed in GUIv2.