When a new node is created with the <kbd>TAB</kbd> key or by clicking the `(+)` on-screen button while multiple nodes are selected, place the new node below all the selected nodes. (Previously, the new node was placed below the node that was selected earliest.)
Additionally, when placing a new node below an existing non-error node with a visualization enabled, place the new node below the visualization. (Previously, the new node was placed to the left of the visualization.)
https://www.pivotaltracker.com/story/show/180887079
#### Visuals
The following screencast demonstrates the feature on various arrangements of selected nodes, with visualization enabled and disabled.
https://user-images.githubusercontent.com/273837/159971452-148aa4d7-c0f3-4b48-871a-a2783989f403.mov
The following screencast demonstrates that new nodes created by double-clicking an output port of a node with visualization enabled are now placed below the visualization:
https://user-images.githubusercontent.com/273837/160107733-e3f7d0f9-0161-49d1-8cbd-06e18c843a20.mov
# Important Notes
- Some refactorings that were needed for this PR were ported from the #3301 PR:
- the code responsible for calculating the positions of new nodes was moved to a separate module (`new_node_position`);
- the `free_place_finder` module was made a submodule of the `new_node_position` module, due to the latter being its only user.
@akavel spotted a compilation error, when building test for graph_editor crate. The cause was that:
* prelude without serde still added serde derivatives in im_string_newtype
* and the graph_editor needs serde from prelude anyway (because it wants to have serializable ImStrings).
