TODO

* Port "select a region" to use cairo to have transparent rubber banding
as in thunar and xfdesktop.
* When selecting a region, Escape should cancel everything.
* Check for leaks using valgrind.
* Capture the mouse.
* Use Xshape to have a transparent background in the generated screenshots.

Rewritte generate_filename_for_uri or remove it. Jannis suggestion:
You can use GDir for reading the contents of a directory and you can use 
regular expressions (GPattern or something) for matching strings against 
patterns, so why not iterate over the contents of a directory and 
determine the Screenshots-%d.png file with the largest %d?

Low priority:
* Try to get borders captured with compiz.
* Consider porting the code to use GObjects instead of structs.