Piano keys in the piano roll doesn't send aftertouch events

### Bug Summary

There's a slight inconsistency between the behavior of piano keys in the instrument window and the piano roll. Both of them support an undocumented feature that the played notes' velocity is determined by the mouse click position on the keys. The further top/left the click position is the lower the velocity gets.

The difference lies in how dragging the mouse over the keys behaves. The keys in the instrument window send aftertouch events to control the value of the currently playing note on dragging the keys. The keys in the piano roll does nothing in this case. I would expect them to work the same way, just for the sake of consistency.

#### Screenshot

https://user-images.githubusercontent.com/1627292/119252156-e4a84700-bbaa-11eb-92c7-5652e1dce845.mp4

#### Affected LMMS versions

AppImage, Version 1.2.2 (Linux/x86_64, Qt 5.9.7, GCC 5.4.0 20160609)
AppImage, Version 1.3.0-alpha.1.102+g89fc6c9 (Linux/x86_64, Qt 5.6.3, GCC 5.4.0 20160609)