Allow locking frames (prevents deletion)

[neilccbrown]

We should add the ability to lock frames so that they can't be deleted without first explicitly unlocking them. The more I think about this the more I think it might be a good idea. The main use case is that the teacher wants to give out some code with an example with pre-provided code that they don't want the student to edit or accidentally delete. This is particularly useful in Strype because you might want to give out code in the main section which is locked, but which calls an editable function. You may also want to give out code which calls a main loop (that updates a set of actor classes periodically on a timer) which you don't want them to edit.

A frame would be in one of two states: locked or unlocked. You can toggle it on a context menu. Locking is like disabling in that it prevents editing (including inside a locked frame), but unlike disabling the code is still active and will be executed.

Plus: this is a feature that is hard to implement in text-editors, but comes quite naturally with frames.

Edit to add: we could also allow locking of slot editing but still allow moving, which would let us support Parson's problems quite well.