HeaderBar non-draggable window issue

[Michael Strauß]

I think I've found a solution that should work for everyone:

1. If you just place controls in the header bar and do nothing else,
you'll get a resize border on top of your controls. With this default,
we the resize border takes precedence over interactive controls.

2. If you specifically set HeaderDragType.NONE on your controls, and
the controls are pushed up to the edge of the header bar, you won't
get a resize border on the controls. Use this if you want the entirety
of your interactive controls to be clickable.

The code is in the latest version of the PR.