Fix default values of window properties not being sent to IWindowImpl

[TomEdwardsEnscape]

This PR adds TopLevel.CreatePlatformImplBinding, a lightweight internal binding system between TopLevel and its platform implementation. This makes it easy to set up a relationship which correctly synchronise values, and also reduces the number of property changed observers flying around.

What is the current behaviour?

Many window implementation properties are only synchronised when they changed. If the default value of the property is overridden, no change event will be raised and the value of the TopLevel and its impl object will be out of sync.

The PR includes a test for this scenario.

How was the solution implemented (if it's not obvious)?

My first approach involved static property changed subscriptions, but I quickly found that this required a lot of type casting. I instead opted for a property-to-action dictionary per TopLevel instance. This allows much terser lambdas to be written with no type casting at all, and makes it trivial to synchronise the values as each object is constructed.

This approach does require regeneration of each callback once per TopLevel instance, but given the special nature of the type (sometimes just one instance is created per application lifetime), I don't see that becoming troublesome.

Breaking changes

None

Obsoletions / Deprecations

None

Fixed issues

Fixes #11307

[grokys]

Looks like it should idiot-proof these bindings, though i'd like to wait for input from @maxkatz6 @kekekeks before merging.