Add attribute in SearchableList.expansion to expand the items by default

[julien-levarlet]

Use case

The current behavior of the expansion constructor, is that the items are not expanded by default and are expanded when the search bar is focused. I would like to have more control on it.

Proposal

I think it would be better to have an attribute to change this behavior and expand by default all the items in the list.
This can be done by setting the initiallyExpanded in the ExpansionTile widget.
I will make a pull request doing that.

[koukibadr]

@julien-levarlet That's a good enhancement I'll review the PR then and include this in the next release