Codergreen's new shiny simplified shop mode with trade support.

The patch is a merge from the codergreen:master with the following
adaptations:
- Removed the unecessary changes to:
  - src/defs.h
  - ValyriaTear.cbp
  - src/common/global/global.h
  - src/common/global/global.cpp

- Reformatted the source to use spaces for minimal changes shown.
- Fixed a few comments here and there.

Please use this patch for review process.
@codergreen: Please, rebase your master or refork and cherry-pick this patch
before proceeding with new changes.

Thanks!! :)