Added (de-) select-all-feature for checkboxes

Hi.

I added a feature that I've been missing for a long time now — the possibility to (de-) select all checkboxes e. g. when confirming the deletion of fields from a template.

Btw: this is my first contribution to ProcessWire and chances are that I missed some important detail, best practice, or the like. If this is the case I'm sorry. I don't want to waste anybody's time here!

Sincerly
Emanuel