Add support for breakpoints to make grid_layout responsive

Removed some bugs.
Most important: If the available grid areas are changed, existing region content is moved to new areas.
Also have simplified and cleaned up the overall code.

-> Since this project isn't maintained, I would be fine to do so!
This type of UI for layout_builder layouts is exactly what i want so i see myself putting continued effort into it.

Because this project seems to be unmaintained and i've changed 90% of the grid_layout in the meanwhile here's a new project drupal.org/project/generic_layout to continue the work.

All the best, TomSaw