[SmartHint] Add opt-in AP to control hint padding brush

[nicolaihenriksen]

Fixes #3700 Fixes #3262

If applying a custom background, the MaterialDesignOutlinedTextBox style by default uses the MDIX window background brushes for the "padding" on the hint (used to "erase" some of the outline border).

This PR introduces an opt-in AP which allows consumers to override this to match their custom background.

In principle, a PriorityBinding should suffice for this in the SmartHint style, but yet again I was unable to make it work 😞 Thus I went for the converter approach I have ended up with before. Apparently I am not great friends with the PriorityBinding.

For a more detailed explanation of the issue, see my comment here: https://github.com/MaterialDesignInXAML/MaterialDesignInXamlToolkit/discussions/3262#discussioncomment-10905148

Before

HintAssist.Background left at default value (Transparent):

HintAssist.Background set to match background:

After

HintAssist.Background not needed unless you want a custom background on the hint (floating and non-floating)