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

Fixes #1290
Caption menu style fix
Improvement of FxMixer multithreading
Use dynamic building of jobqueues with dependency counting:
- At the start, each channel that has no dependencies is added automatically to the queue
- Then, after each channel is processed, it increments the dep.counter of all its recipients
- When a channel's dep.counter hits the amount of its dependencies (senders), it gets automatically added to the queue
- The queue is finished when the master channel has been processed
- Muted channels are automatically processed at the start regardless dependencies, because they don't have to care about senders, being muted

Hopefully this will improve Fx Mixer performance.
Merge pull request #1299 from LMMS/stable-1.1-iss1290

Disable 'Help' action in context menus when there's no help text
Fix segfault when enabling/disabling sends while playing
More updates to FxMixer - better handling of muted channels
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