Grab/release UI commands

Conceptual command:
- Grab
mouse-down (PC)
touch-down
connect two fingers or press and hold controller (VR)
- Release
mouse-up (PC)
touch-up
disconnect two fingers or release controller (VR)

If grab/release commands happen over an existing link, this makes the value equal to its ID.
If grab/release commands happen over an empty space, this makes the value equal 0 (zero ID, the sing of ID absence).

For example:
* We can grab an existing link's center and release it on another link's center. This will make a new link connecting selected two.
* We can grab and release in the empty space making both values reference to newly created link, making it structure as `{ id: x, from_id: x, type_id: 1, to_id: y }`, the default type should be selected in the UI (maybe in the settings), otherwise it is 1 (the type).
* We can grab, move and release in the empty space. This will make a floating link with a size exactly as the motion path.
* And other combinations of these examples.

Grab/release commands can be used not only on the link's centers, but also on their ends, making it possible to update/recreate links with new structure (from_id, type_id, to_id).