Mixed state checkbox example does not correctly recall previous mixed state in some scenarios

[mcking65]

The mixed state checkbox example does not correctly recall the last mixed state of checkboxes if the mixed state was created by changing the state of controlled checkboxes from the all checked or no checked states.

The code should record the set of checkbox states any time the master checkbox is in the mixed state, regardless of what event caused it to get to that state. Then, restore that state any time the master checkbox is in the unchecked state and it is activated to go back to the mixed state.

Example tests 1:

1. Load page.
2. Check master all so all boxes are checked.
3. Uncheck tomato so master is in mixed state.
4. check master to return to all checked state.
5. Check master 2 more times; should return to state in step 3.

Example test 2:

1. Load page.
2. Check master all so all boxes are checked.
3. Check master so no boxes are checked.
4. Check one or more boxes so master is in mixed state.
5. check master to return to all checked state.
6. Check master 2 more times to return to mixed state; should return to states in step 3.

Issue reported by @racer2207 and @annabbott in issue #471.

[mcking65]

This is fixed with ca272bf and thanks to @gerardkcohen!