[guilib] Drop back compatibility code and inconsistencies

This drops some old backward compatibility code for skins.  The aspect ratio change and label positioning changes are kept but are now switched based on the API version the skin requires, allowing a bit of time for skinners to adapt (the skins still work, but they look a bit odd in places).

I'm fairly sure no skin is using the old context menu stuff (context menu without a grouplist), so didn't bother with backward compatibility there.

@ronie, @BigNoid, @Black09, @JezzX please feel free to comment.  Confluence will need updating for the label change (labels in lists now have `<left>` meaning left) and the aspect ratio change (all images default to `stretch` now - before infoimages in lists defaulted to `keep`).

Closer to release of Helix we can decide whether to drop the backward compatibility completely or not.  e.g. if there's other, breaking changes made.