feat select: add hover delete for selected items in multiple

To deselect an item in a select multiple, there was no other solution than opening the
dropdown, searching the item and unselecting it. By adding a button when hovering the
selected item, we can now delete it more easily.
feat select: deselection on non-multiple selects

Add `allow-clear` option to be able to remove the value from a non-multiple select.
When an item is selected, a cross is available beside the dropdown arrow. Clicking on it
remove the currently selected item.