Add ability to create child nodes.

The focus right now is to expose the core editor functionality via the
right-click context menu.

The root node of the current tree can be created by right-clicking
within the editor window and then selecting the corresponding entry from
the context menu.

A child node can be created by selecting the parent node,
right-clicking at the location where the new node should be created,
and then selecting the corresponding entry from the context menu.
Ignore any .meta files Unity creates in src/editor.