[FEATURE] Validator for multi-select and multi-checkbox inputs

[rr-it]

The validator design takes account of text-inputs only.

For multi-select and multi-checkbox inputs it might be usefull to validate like:

• At least 2 options are selected.
• A maximum of 3 options are selected.
• At least 1 and a maximum of 5 options are selected.

Would you mind to accept validators like these to be added to the powermail core funtionality? Or are these to be implemented by extending powermail in a custom extension?

[mschwemer]

Currently we try not to add features to EXT:powermail. We call it basically feature complete, as we must maintain all additional code also in the future.

If you provide it as a PR, we will definitly have a look into it. The other (faster) possibility is, to provide an extending extension.