Adding DependencyProperty for ShouldShowOnKeyboardFocus

[batzen]

Fixes Issue #4172

Description

Converts the virtual internal property for ShouldShowOnKeyboardFocus to a public property and also adds a DependencyProperty for it.

Customer Impact

Tooltips always show on keyboard focus which is not the desired behavior in all customer applications.

[batzen]

ApiCompat will complain about the changes i made.
I don't know what has to be done to stop ApiCompat from complaining.

[batzen]

IMHO, this should also be backported to .NET 4.8.

[SamBent]

@batzen - There is another round of tooltip accessibility changes on the way. We will include a proper solution to the extensibility issue you raised in #4172. (Your solution here isn't quite flexible enough.) So I'm closing this PR.

Changes to the public API surface cannot be backported to 4.8. Unfortunately that rules out any solution that introduces a new property or method - yours or mine. We'll add the extensibility point in .NET 6.0, but older versions are stuck with the current behavior.

[batzen]

@SamBent Could you be a bit more specific on why my solution isn't flexible enough?

Is there a chance that Microsoft could be more open/transparent about such upcoming changes?
Currently it feels like Microsoft alone decides what and how additions to WPF happen, and that doesn't feel very good to be honest.

Just to make sure i understand this correctly:
Microsoft changes the behavior in .NET 4.8, making it the new current, and says "now you are stuck with that as it's current"?
If behavior in .NET 4.8 can't be changed, why was the behavior changed in .NET 4.8?

[SamBent]

See #4242