[LVGL] make start / end angle of an arc widget available as literal / assignable / expression

[jcuypers]

Make it possible for a user to either set the arc via value or via the start and and angle. In order for this to work, start and end angle of the arc widget need to be assignable and/or to be set through expression

It would be possible to build this up after code generation, but it would be nice to have the same workflow everywhere