Created instances of Grid should be an array

Panes should be added by pushing them into the array, removed/expelled by popping or shifting them, indexOf should work by id, etc.

Start by locating the Grid instance implementation and any existing pane-management tests; the issue names no files or entry points. Check how panes are currently stored and manipulated, then define completion as Grid instances supporting array operations and id-based indexOf behavior, with tests covering those operations.

Written by the indexing model from the issue text.