Previously if the menu was opened and no there were no options to be
displayed it was just showing a tiny empty menu, this commit adds an
empty placeholder in order to make it clearer that the list is empty.
Rework a bit the recent work made on RHF example to make some component
more generic, such as RhfSelect and RhfDatepicker, which is based on a
design using RHF context hooks to provide a seamless integration.
Fixes#144
Adding this new variant makes necessary to reorganize the files to keep
a clear separations of concerns. As of now Select/index.tsx is just an
entrypoint to render either the mono or multi variant of the select.
