Avoid collisions on entity insertion

[nkrkv]

Rationale

Currently, if I place a node or comment with i key, (+) button, or with Edit → Insert XXX (i.e., in all cases when the desired position is not specified), the entity is going to be inserted somewhere in the top left corner. In many cases, it would place right onto/under an existing entity.

It makes harder to notice the result of the operation and to pick it for movement to the desired position.

We could check insertion position and if it is occupied, find the first free slot to the right.

Acceptance criteria

• [ ] If an entity default position is occupied by another entity, the first free slot to the right of the default position is chosen
• [ ] The mechanics work for all comment/node combinations
• [ ] The mechanics work regardless of whether the existing entity is exactly at the position or it is placed to the right but has a multiple-slot width
• [ ] The collision check takes a width of the entity inserted into account