Multi Instance: Refactor: Split RootNode And Explorer

[alex-courtis]

• Multi Instance - Contributors Wanted - Please Take An Issue #2255

Requires #2916

Explorer is a RootNode. Break them and store RootNode in Explorer.

get_node_at_cursor returns the root instead of fake ".."

Replace name == ".." tests with BaseNode:is(RootNode)

Change name to be consistent with DirNode

[alex-courtis]

This may not be necessary; there are no conflicts with the nodes.

RootNode may be unnecessary and Explorer could inherit DirectoryNode