Node manipulation by level

[stengel]

Rather than clicking on single nodes for adding nodes, setting players, merging / dissolving info sets (?), allow this operation for the whole level by having a dashed line appear when the mouse hovers over that level, and clicking on that line applies the operation to all nodes indicated by that line.

Specific behavior when adding nodes:
In principle, clicking on that line for "apply to all nodes on that level" just means performing the "add" operation to each single node. However, I think this would be confusing in the following scenario: Suppose the user has first generated children by clicking on individual nodes, and is now on the fourth level with 8 nodes and is getting bored. After giving individual children to the first two of those 8 nodes, he discovers the "level" functionality and clicks on the line. Now, simply applying "add" to all children would result in the first two getting an extra child (now having 3 nodes), with the remaining 6 getting 2 children, which is most certainly not the intention. I think the effect (number of children) should be uniform for all nodes on that level. This would be implemented as follows:

• find the node with the fewest number of children on that level
• for all nodes that have that same (lowest) number of children, apply the "add" operation. In particular, nodes that have already several children (e.g., three children), are untouched.
• the effect is that if all nodes have the same number of children, they are all treated the same way.
• if the result is not as desired, the user can simply use the undo button.