fixes for custom page size settings

- issue with Custom displaying as A4 is due to the fact that code assumes Custom is first item in list

- issue with margin set incorrectly a result of failure of code to update the maximum margin after change in width

Also changed it so that merely selecting "Custom" in the dropdown doesn't actually change page size - it's kind of a no-op.  Currently, it sets page size to 1x1mm, which is pretty useless.  LibreOffice leaves the size unchanged when setting page size to User, and I thought that made more sense for us too.