Allow nodes to be dragged on to empty cells

[F1nnM]

Nodes can be merged now. Merging nodes A and B only works, if at least one node doesn't contain text (is empty). Merging can be done by dragging:

• an empty node onto another empty node
• an empty node onto a text node
• a text node onto an empty node

Dragging the base of an arrow to the target of it, turns the arrow into a loop.

Everything seems to be working fine, but I'm not 100% sure I caught all bugs, so tell me if I introduced any bugs.

[F1nnM]

I think I fixed everything

[F1nnM]

Hi, I really don't want to pressure you, but I also really would like this PR to count towards hacktoberfest, so if you could accept it before the end of the month, that would be great! 😄