[X] When (un-)checking *all* checkboxes of siblings, toggle parent checkbox

...or something like that.
I'm not sure if that was the desired behavior, but I made parent checkboxes checked if and only if all children were checked
Also, if you (un)check a checkbox, all children get (un)checked.
Hope you are ok with the refactoring... and if you find bugs, I'll try to fix them.