Initial commit for semi-stable pqiv 2.0 -- Release 2.0-rc1

This is a complete GTK 3.0 rewrite, backwards compatible to GTK 2.0. It
uses Cairo for drawing and is therefore much faster. Other features
include preloading of images for faster image switching. Furthermore, I
make much better use of GTK's event system, resulting in improved
compatibility.

The rewrite has now reached a quite stable state. It runs smoothly in
Mutter, Muffin, i3, xfwm and Mingw32. I will therefore stop using the
development branch and begin versioned releases. Please help test this
version and report bugs!

This rewrite is not entirely backwards compatible, especially the
configuration file format has changed and some of the command line
switches and configuration switches are gone. Therefore the major
version jump.

I plan to release a stable version in about a month or so.