Options fixes

Dave,

This primarily makes the change I mentioned to the new Screen Width UI: It moves the unit label (mm) to *after* the number input instead of being part of the label, for consistency with the rest of the new options layout. I have also made some minor tweaks to the layout of the option.

I have also taken the liberty of renaming the new option to "Physical Screen Width"; I think this is clearer than "Screen horizontal size". I hope you agree.

In the second commit are some fixes to the "temporary" fake sizers I used while designing the other options pages, but forgot to remove at the time...

Caesar

PS - I have found a way to correctly read the screen size on OS X. I will send a separate pull request when I have implemented it fully.