Easier deletion of blocks

Summary:

Allow deleting a block by clicking its handle on the left
and pressing DEL, making editing much easier.

Current behavior:

To delete a block, you must precisely select its contents, which is fiddly
Placing the cursor at the start of the block is tricky because it changes to a 👆 cursor near the handle.
After selecting and pressing DEL, the block remains and requires an additional BACKSPACE to remove.

Suggested improvement:

Clicking the block handle and pressing DEL should delete the entire block.


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