Adwaita: Improve columnview styling

This merge request does the following:

• Fix separator color for lists.
• Move padding from parent row to child cell.
• Align horizontal sizing of cell with header button.
• Properly support GtkColumnView:show-column-separators.
• Change cell height with and without .data-table.
• Don't restrict columnview editablelabel style to .data-table.

Before

The child labels can appear to be connected:

After

The child labels no longer appear to be connected:

When show-column-separators is TRUE:

When show-column-separators and show-row-separators are TRUE:

When .data-table is NOT set:

CC @jimmac