Re-work scaling following excellent insights by Carsten Kurz, described

on #1669.  Now the scaling of video content is guessed based on its
aspect ratio (taking into account pixel aspect ratio) and there is a
"custom" option for overriding incorrectly-marked-up content or content
which actually needs its shape to be changed.

Essentially we are defaulting to what used to be called "no stretch".

This commit also adds buttons to change left/right and top/bottom
crop together, and auto-guessing of DCP container size when any change
results in there being just one piece of video content in a project.

These changes include breaking changes to metadata.xml; old projects
should load (with new settings that preserve the project's apperance)
but once saved they will not load into 2.14.x.  DCP-o-matic saves a
backup file (metadata.37.xml) when 2.15.x loads an old project.