Add proper Drag-and-Drop and Clipboard support

Currently, these two features are quite limited in baseview (or not implemented at all, in the case of Clipboard).

Most notably:

There is no concept of "sessions" (WindowHandlers have to re-read the entire data on every event);
Reading the same data in different formats is not supported;
The asynchronicity is currently hidden from implementors (so they cannot reject a DnD operation early for instance);
There is no support for starting a DnD/clipboard operation from baseview;
Probably more

This work would likely integrate #257 and #258, and also likely require multiple breaking changes to the current API.

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