Web\Form: relax form submission check

We have to live with some badly designed forms right now. Some have
submit buttons but don't know about, others don't have such but link
to foreign controllers fiddling with the form and adding different
button AFTER the form got submitted - it's a mess.

Relaxing the submission check to "just check whether btn_submit has
a value" fixes most of this. However I do not consider this being a
solution for the long run.

fixes #6423
fixes #5967
refs #6540