Don't display 'Help' action in context menus when there's no help text

Fixes #1290
Caption menu style fix
Merge pull request #1299 from LMMS/stable-1.1-iss1290

Disable 'Help' action in context menus when there's no help text
Add a method to track value changes in AutomatableModel - this can be used by DSP code instead of signals/slots to improve performance
Modify BassBooster to show how to replace signals/slots in DSP
Merge pull request #1307 from diizy/model

Add a method to AutomatableModel that allows tracking changes without using signals/slots
Merge pull request #1305 from diizy/stable-1.1

Improvement of FxMixer multithreading
Fix bassbooster