Locked point annotations can still be edited

[petebankhead]

Bug report

Describe the bug
Locking a point annotation doesn't have any impact on whether it can be edited or not. It's still possible to add or remove points.

To Reproduce
Steps to reproduce the behavior:

1. Open an image
2. Starting annotating with points
3. Lock the point annotation (e.g. through the ellipsis button under the 'Annotations' tab)
4. Continue adding points

Expected behavior
If an annotation is locked, then clicking on the image with the points tool active should start a new annotation (similar to how the brush/wand behave).

Desktop (please complete the following information):

• OS: All
• QuPath v0.3 (and presumably before)

Additional context
Needs to be fixed for v0.4, where locking is more evident the current behavior is surprising.