Revamp repo structure and build system

[dequis]

The whole "check out from mercurial and patch it for libpurple 2.x" thing that this repo is doing is confusing and discourages contributions.

A while ago I hired an intern to help here, and there is a branch called "libpurple-integration" (i didn't pick the name) with steps towards that which merges the relevant hg history of the upstream plugin into this repo.

Sadly it has reached the point where it's my responsibility to review these changes and integrate them, and I can't.

I need someone with maintainer mindset to do the rest of this thing.

Steps forward would be:

• review the commit history
• do any required history rewrites to clean things up
• turn the "patches" directory into real commits (ideally maintaining author metadata)
• modify the build system to not do patching anymore