copy graphic assets from fdroiddata and app source into repo

copy graphic assets from fdroiddata and app source into repo

This looks for standard graphics assets in two standardized locations, one
in metadata/ subdirs and another in fastlane's standard dirs in the app's
source repo.  When it finds them, it copies them into the repo, where they
will then be included in the index for use by fdroidclient.  The images in
the metadata/ folder of fdroiddata take precendence over the files in the
app's source repo.

So like this for including graphics in fdroiddata:
  metadata/packageName/locale/filename.(png|jpg|jpeg)
for example:
  metadata/org.videolan.vlc/en-US/featureGraphic.png
or
  metadata/info.guardianproject.ripple/zh-CN/phoneScreenshots/screenshot1.png

Including graphics in fdroiddata would be optional. The prefered way to get
graphics into the repo would be for the files to be in the git repo in a
standard location.  This fastlane layout is currently supported:

https://github.com/fastlane/fastlane/blob/1.109.0/supply/README.md#images-and-screenshots