F7.4 improve detection of vias

[poeschlr]

Vias have different layer requirements compared to normal through hole pads. This means they need to be handled separately

A first limited implementation was added with #288 but it has some limitations that stem from the chosen design:

• Reliance on the footprint name for identifying possible exposed pads with vias
• The largest (integer) pad number is assumed to be the pad that holds possible vias
  • Footprints with non integer pad numbers are ignored.
  • Different pad numbering conventions (example the use of pad 0 for the EP) are not supported
• Does not support handling pads other than exposed pads

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A possible replacement could work as follows:

• split pads in three lists (smd, tht, non copper)
• check all tht pads if they overlap any smd pad.
  • Mark such a pad as a via.
  • Check if it has the same pad number as the smd pad it touches.
  • Check if it has at least "all copper" layers active.

The hard part of this implementation is finding a way to check if a pad is overlapping another pad. A simplification could be to only consider circular vias and rectangular (or roundrect) smd pads. This would already cover most of the cases found in the official library.
A later extension to supporting other pad types for the SMD pad could be split into another issue.