Use bostree instead of GTree

This commit also reverts d983ef5d7e7b45a39508e35fbd703c0ccac2f16b, the
improvement on absPath's memory usage. The changes are to be
reintroduced in a later commit.
Use bostree instead of GPtrArray
Use pointers to the current BOSNode instead of image IDs
Lazy loading (Load in a separate thread, display window when the first image becomes visible)
Delete files from the tree if inotify informs us about deletion (See bug #7)
Use weak references when passing around BOSNodes

This is required because a thread/callback might be invoked after an
image has already been removed from the tree. The implementation is very
simple: bostree got two new functions, bostree_node_weak_ref() and
unref, respectively. They increment and decrement a counter, as a normal
reference counter would do. The references are weak in the sense that if
a node is removed, it becomes invalid and the weak reference will
resolve to NULL.
Added the -l, --lazy-load option (See bug #11)
Free the keys from the search tree before deleting it
Added --low-memory option to disable image preloading / caching of image surfaces / search tree
Bugfix: Adjusted Makefile for bostree to compile under Mingw32
Image loading: When prev/next change due to a new image inserted into the tree, unload the old, preloaded images from memory
Save some memory even if not using --low-memory

Path strings were always allocated MAX_PATH, but never reallocated. I
fixed that.
Detect nested symlinks through a separate directory tree

This became necessary because the file tree stores the images directly,
so directories could no longer be inserted into that tree. As a side
effect, --low-memory can no longer lead to a symlink-loop, only to
duplicate images.
Remove file_sorted_tree

Now that the nested-symlink problem is solved via a separate tree, the
only improvement brought by the sorted list is to help users who
unintentionally specified a parameter twice or who symlinked an image
multiple times. But both cases admit intentional use, and we prevented
users from that before. Therefore I think removing this feature is a
good idea.
Do not traverse into directories for which realpath failed
Remove some debug-leftovers from the lowmemory / bostree changes
Typo in Readme