Merge `Responder` into `View`

[tig]

For now it is split, in case we want to introduce formal view controllers. But the design becomes very ugly.

Mostly done. All that's left is

• Move Dispose stuff to View
• Nuke IsOverridden - Should be done in Fixes #2489. Create new ScrollBar based on a new Scroll and remove ScrollBarView/ScrollView #3498 by @BDisp

[tig]

This was prematurely closed.

[tig]

This was fixed.