Add a way to show a PurpleStoredImage in libpurple

[seirl]

libpurple abstracts the way clients receive an image via the different protocols by storing it and giving us a PurpleStoredImage struct:

​https://developer.pidgin.im/doxygen/2.5.0/html/imgstore_8h.html

If we want to be able to show these images to the users, I can see two ways of doing so:

• Just display the filename. Only works for people who run their bitlbee instance on their local computer though :(
• Implement an http server in bitlbee to serve the images and display the link (...)
• Leave the http server responsibility to the user, and display an URL formatted like the user configured it.