Add CellCollection for use by Grid

[dhardy]

• Rename GridChildInfo → GridCellInfo
• Add trait CellCollection, macro cell_collection!
• Some fixes

[dhardy]

This fits with other recent changes. Unfortunately, this particular change doesn't achieve much:

• It enables usage of kas::widgets::Grid with varying child widget types, however in this case cells cannot be added/removed and thus the existing grid! macro could be used
• The grid! macro could, when used as a stand-alone macro, be replaced by Grid::new(cell_collection![..]). However, when used as part of the layout of a custom widget, it cannot be.