Experimental poppler/PDF support
Poppler < .22 support -- very hack'ish, not for inclusion in master branch!
Added FILE_TYPE_PDF to distinguish PDF files, support for multi-page PDFs
Replaced Poppler < .22 code with a neater™ version
Refactored most image functions into file type specifics (Work in progress)
Completed Refactoring / temporarily disabled poppler code again

All image backend related functionality now lies in separate functions,
such that multiple image backends can be supported. I intend to add
all backends supported by Evince; at the very least configurable,
but maybe even using a plugin structure, such that a package for pqiv
does not have to depend on all the backends, and different backends
can be served in different packages.

Even without the different backends, these refactorings are a huge
improvement of code quality and helped me find some bugs already.

TODO: Images do no longer have to store an image surface, because
some backends might not need one (say, poppler). Therefore, the external
image filter functions can no longer replace that on the fly. I
currently add a new, memory file after the current image with the
filtered output. The problem with that is that this image stays in
memory until pqiv quits. It probably should auto-unload or similar.
Replaced a G_LOCK I've missed by D_LOCK
Reimplemented the Poppler backend

This is still wip: I don't intend to add a permanent dependency on
libpoppler without having at least a switch to turn it off.

But it neatly shows where this is going: 78 lines of code bring PDF
support, and they aren't any longer spread across the file, but one
block of code.
Bugfix: Force image backends to draw exclusively within their propmised bounds
Poppler: Fixed display of PDFs and memory files handling
Removed old code
Poppler < 0.22 support
Fixed Windows support for latest changes
Reworked Makefile/configure
Fixed windows build with new Makefile/configure
Simpler solution for the previous commit: Don't single-expand the variables..
Moved backends into their own c files and made their inclusion configurable
configure: Made configure aware of other Make systems -> Added check for GNU make
Documentation regarding backends/poppler
libspectre backend for .ps/.eps support