Add setting for default icon for icon list block

[bfintal]

Add a new setting for changing the default icon for the icon list block:

Default Icon for Icon List
Sets the default icon used when adding a new Stackable Icon List block. Existing Icon List blocks are not affected.

Place the setting in the admin Stackable settings under Editor Settings > Block Defaults, (this is a new section, add this below Block Widths).

This setting should show an icon / svg picker similar to the icon picker in the inspector, that users can use to pick the default icon, or upload another SVG.

After changing this setting, creating a new icon list block should use the icon set here.

Things to watch out for

• The design library should not be affected
• Inserting designs from the design library should not produce errors
• Existing icon list blocks should remain the same and not produce an error
• Creating a new entry in an existing icon list block may or may not use the new icon