Previously the group-tab extended all the way down to the
last track. Potentially with a widget height of > 2^15 px.
This caused issues with gtk widgets (notably on Windows)
```
unhandled exception (type std::exception) in signal handler:
what: invalid value (typically too big) for the size of the input (surface, pattern, etc.)
```
This also prepares for Mixer Tab-Group to use the same
separation between visible and total extent.
