Add property which allows header view to handle touch events.

[deRonbrown]

Previously, the header view could not accept user interaction. By setting the mAllowHeaderTouchEvents flag, the margin view will pass touches to the header view to be (possibly) handled.

Common use cases are if the header view contains an ImageView which needs an OnClickListener or if you have a more complex header view with buttons or other clickable elements.

[smbarne]

+1 - this is useful and I've pulled this into my codebase

[MariusVolkhart]

:+1: from me also. Also have included it in our codebase.

[xkokushox]

It worked great for me XD thanks a ton

[doodlleus]

excellent work, thank you!

[jkwiecien]

+1, please merge