Add minItemCount for multiple + required selects

[Geolim4]

Is your feature request related to a problem? Please describe.
Not really

Describe the solution you'd like
I think that for select MULTIPLE and REQUIRED the option minItemCount would be great to have.

Describe alternatives you've considered
No idea ATM

Additional context

User stories:

• The user cannot remove less than minItemCount from selected items.
• removeItemButton is impacted: The removal of selected option(s) is disabled if it equal or less to minItemCount
• Also the option removal using inline DEL keyboard key would also be disabled.
• If minItemCount is equal or greater than maxItemCount (if minItemCount > -1) an error is thrown (even if $silent is enabled ?).
• The option minItemCount is ignored if the select is not MULTIPLE and REQUIRED.
• Implicitly a minItemText option must be created in case the user attempts to get less than minItemCount option
• By default minItemCount value is fixed to -1

[Xon]

This would require #1148 to be addressed

[zoglo]

This would require #1148 to be addressed

That could be implemented by #1332 :)