make behaviors usable in styles

[bitbonk]

In the past there has been a lot of confusion and frustration about the fact that behaviors cannot be used in styles.

Now that WPF is open source, I‘d like to suggest that we get rid of this odd limitation.

Being able to use behaviors in styles would be very helpful.

[batzen]

As a reference: https://github.com/MahApps/MahApps.Metro/blob/72f69b37de10231e55e9eaa9838faf32a8355998/src/MahApps.Metro/Behaviours/StylizedBehaviors.cs

[shadowaltar]

@batzen since you've mentioned mahapp (i used it in pet and company projects) there are a lot of custom controls here and there could have been added to Wpf arsenal :)
Don't know why previously ms said wpf is "feature complete".