Add allowEmpty property to SelectButton

[jnoordsij]

Describe the bug

In #3610 and #3708, the behavior for the unselectable was reversed. This is very confusing, as it is dubious wether or not this is an improvement to the logic.

Moreover it certainly is a breaking change of behavior and should not have been added in a minor version update.

For further reference, please see #3708 (comment) and below.

[rubjo]

I really cannot see how this is dubious at all, the behaviour reversal was unfortunately just wrong.

All of these are logically the same:
unselectable
possible to unselect
possible to clear
clearable
can be blank

If another wording is less likely to be misinterpreted, I agree with @jnoordsij that this should be fixed as per his suggestion in #3708.

[kennyfellows]

Agreed. I had to read the code to figure out why I needed :unselectable="false" in order for it to be unselectable.

[colinstevens06]

Agreed. Is not intuitive as it currently is labeled. Now that I know, I know, but I expected it to perform the opposite of how it does based on the naming of the prop.

[jnoordsij]

This has been adressed in #4107, by deprecating the old flag and introducing a non-ambiguous new flag. Would be great to have it merged!