Add a "hasPlaceholders" attribute on the grid in design mode
More tests and doc on the grid component
Add a setIds method on the Manipulator

Each node in the given grid (including the main node) will have an ID
composed of its tagName, a dash, and a unique id
Add a method in the store to get a grid node by its id
Pass the grid to the component, not its name
Use textContent instead of innerText
Don't update existing IDs
Add a mockUniqueId test util

Will allow to check real unique Ids but with the internal counter reset
to 0 in each test function.

Also changed the setIds method of the grid Manipulator to respect the
tree order of manipulated nodes (because querySelectorAll returns nodes
in tree order, the main node should come first to respect this order, so
the uniqueIds are in order)
Add subgid/row/cell components