Throttle function not updated on edit

[gfgit]

Describe the bug
Throttle widget does not react to decoder function edits.

To Reproduce
Steps to reproduce the behavior:

1. Create a decoder with no functions
2. Open throttle
3. Add a function to decoder
4. Throttle widgets does not show new function
5. Close and reopen throttle widget. It now shows the function

Expected behavior
Throttle widgets auto updates.

[reinder]

That is missing indeed, I'm not yet happy with the current throttle widget, it was mainly build for testing decoder control, but for train control there is more needed, I'll move this one to the v0.4 milestone.