"(De-) Select all" 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 deleting fields from a template.

Btw: this is my first contribution to ProcessWire so chances are that I missed some important detail, best practice or whatever. If this is the case I'm sorry. I don't waste anybody's time time here!

Sincerly
Emanuel