Automatically reload an image when its file was changed

[sdx23]

This behaviour is common in many other applications e.g. pdf viewers but also in other image viewers. It can be useful when modifying or generating images by a script (for example working with gnuplot).

The setup is such that different backends for notification about file changes are supported and can be selected at compile time. A dummy backend and a backend using inotify are included.

Automatic reload can be disabled by a command line option.

[xyb3rt]

Thank you very much for your contribution. It is very welcome as it fixes a pretty old issue, for that I only had a very cumbersome workaround.

I decided against picking the second commit. I do not think that an option to disable autoreloading is that useful.

Please do not take it personally that I changed so many things. I've tried to split the changes in as many commits as possible, so that you can understand the motivation behind every change.