Forms: fix unclicking standalone form buttons
We can find pdf forms that eg. have three related radio buttons where their *only* relationship is having the same full qualified name. Such a form is supported by Adobe Reader and produced by LaTeX through TeXStudio. So, when clicking one such radio button, Poppler failed to unclick the other radio buttons as they are not children nor parent of the clicked one (which is the ordinary way to set radio button groups as per the PDF standard). So, when clicking a radio button, let's add support to find other radio buttons in the same page that are only related by having same fully qualified name (these may be in standalone or normal fields). Where trying to find where PDF standard covers this special case of related radio buttons, the closest thing we can find is section "Field names" inside chapter "8.6.2 Field Dictionaries" in 1.7 PDF spec.
Closes #1034 (closed)