Add BeforeEndDrag and EndDrag event

[silentgrant]

Hello, I'm a novice at GitHub. Forgive my rudeness please.
I need handle something before EndDrag event. So I add BeforeEndDrag event that is called before EndDrag. It also support EndDrag event for DockContent.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}