Rename uploaded files, prevent overwriting of existing files, configurable fallback image and code cleanup

My next run on the code ;-)

1. Uploaded files can now be auto-renamed. There is also a maintenance module to mass-rename existing avatars.
2. Prevent overwriting was not enabled by default, it is now enabled via DCA.
3. The fallback image is now configurable.
4. In the registration of modules and hooks, I remove the developers namespace to make the contao "class overwrite" feature available.
5. Do some code cleanup, most is just reformatting.
6. I added myself to author and copyright notice ;-)