Re-use existing unlink command and move all the unlinking code to
NodeRemoveCommand to cover all bases (deletion of a block in the
timeline or in the node editor).
When a Block node was deleted its linked blocks were not unlinked
which caused a crash when the user tried to select a linked block.
This fixes the issue by ensuring all links are removed when a Block
node is delete.
Undo/redo is also supported.
This required various changes to various parts of the infrastructure (mostly
using paths to finding the "root project" of any given object throughout).
Now theoretically infinite projects can be opened and accounted for at any
given time.
Previous iteration used some "magic code" that added clips automatically to the
timeline. This was functional but ultimately outside of the undo commands'
control meaning nodes could be infinitely added and abandoned. This makes the
add process part of the undo command which means it's all undoable as the user
would expect.
Thomas Wilshaw