Functionality in combo box' context menu doesn't make sense for most combo boxes

When clicking on the arrow of a combo box they all show a generic menu that enables to connect the combo box' model to a controller but in most cases it does not really make sense:

The problem can be reproduced as follows:
1. Start LMMS.
2. Add a new LFO controller.
3. Right click on the arrow of the zoom control in the Song-Editor.
4. Select "Connect with controller..." (or however it is called in the English version).
5. Select "Controller 1" and press OK.

We have now automated the zoom control to make our session more exciting. :smile: There are also several combo boxes in the Piano-Roll and the Automation-Editor that can be automated this way but where it does not really make sense.

Please also note that the value for the combo boxes is decreased before the context menu is shown.

Start by reproducing the issue in LMMS with the Song-Editor zoom control, then compare the context-menu behavior of combo boxes in the Piano-Roll and Automation-Editor. Trace the shared combo-box context-menu path and identify which controls should not expose controller connections. Done means inappropriate menus no longer appear and opening the menu does not unexpectedly change the selected value.

Written by the indexing model from the issue text.