Windows build: Added plugin_replacement.py script for replacing unchanged
plugins with original versions.

- Performs the following steps:
    - Downloads current plugins from the public plugin server and determines
      their build dates
    - Determines for which plugins cpp or pro have changed since build date
    - For any plugins that haven't changed the matching (old) pdb file is
      retrieved from the public symbolstore
    - Any newly built plugins that didn't have any change detected are
      replaced with the old dll and pdb versions
- Requires the following python modules (all available via pip):
  pefile, requests, gitpython