Enhance Flexibility: Allow Custom Widgets alongside Icons in SlidableAction

[mohammedzizou]

The modification to the SlidableAction class allows it to accept either an icon (IconData) or any custom widget (Widget) as its leading element,ensuring more flexibility in design. Additionally, assertion checks were added to prevent both from being used simultaneously and to guarantee that at least one visual element is provided.

[antonderevyanko]

This PR is definitely needed.