Have one write_properties() function

[lwjohnst86]

Having looked at these functions a bit, maybe a natural organisation would be:

• edit_package_properties (leave as is):
  • takes a partial PackageProperties
  • returns a complete PackageProperties, ready to write to file
• edit_resource_properties (= most of the current write_resource_properties):
  • takes a partial ResourceProperties
  • returns a complete PackageProperties, ready to write to file
• write_properties (= the current write_package_properties)
  • takes a complete PackageProperties

The big change here would be that edit_resource_properties would return a PackageProperties (with the ResourceProperties updated or added). This gives a nice flow, but I'm not sure if it's misleading?

Originally posted by @martonvago in #990 (comment)