Add pre-hook for properties

[TimvdLippe]

Add pre-hooks for properties to validate and transform properties.
Adds a new field to a property called set that references the name of a function. This function takes a value and returns the new value.

Fixes #3099

[TimvdLippe]

Fixed a stack-overflow issue when a child notified an binding update.

[BLamy]

👍 This is a lot cleaner then my original proposal.

[kaste]

Don't you actually need this lock/flag only b/c your setFn's in the tests are actually not idempotent?

[kaste]

Shouldn't this be wrapped by a try () finally ( ...releaseLock... )?

[TimvdLippe]

The stack-overflow error can be triggered quite easily. If the parent has defined a property with a setter and binds this to a property of a child, when the child property is updated the stack-overflow error is created.

Actually I was thinking of leaving this change out (so only pre-hooks, no transformation magic) as the cons might outweigh the pros.

[sorvell]

This PR may fit this category: #3369.

[kevinpschaaf]

With a feature like this, there will be a high temptation for users to depend on other element properties (e.g. this.min, this.max, etc.) in their validation logic, but this provides no way to declare those dependencies to ensure that a.) the values have been configured or b.) the validation is re-run when the dependencies change.

As-is, I don't think this would solve enough use cases to warrant an addition to the API surface right now. Computed properties solve much of this problem space, both the dependency issue and the efficiency issue related to taking action on the unvalidated value, by differing the value on the API from the one the element actually uses internally. As has been well stated, this comes at the cost of extra property definitions which we understand can feel burdensome. It's a hard problem so we'll continue to consider ways to make this pattern easier.