add support for disabling auto-fill feature

[pixelzoom]

2/6/20 design meeting. From #92:

• There's a feature that autofills the beaker at startup and reset, or when a new solute is selected. It's used in ph-scale, but not in ph-scale-basics. Do we need to be able to disable this feature?

Decision: Yes. Add autoFillEnabledProperty

[pixelzoom]

Added:

phScale.macroScreen.model.autoFillEnabledProperty
phScale.microScreen.model.autoFillEnabledProperty

To test:

1. Start sim in Studio
2. Select a solute from the combo box, note that the beaker automatically fills to 0.5L
3. Change autoFillEnabledProperty to false
4. Select a solute from the combo box, note that the beaker remains empty
5. Press Reset All button, note that the beaker remains empty
6. Change autoFillEnabledProperty to true
7. Select a solute from the combo box, note that the beaker automatically fills to 0.5L

@arouinfar please verify in master.

[pixelzoom]

QA will need to know about this issue. So when it's verified, please change label to "status:fixed-awaiting-deploy".

[arouinfar]

Looks good in master @pixelzoom.