Move output to the "./bin/" directory

This changes the project files and the build scripts output directory to a new "bin" directory instead of the tree root.

Notes:
* Needs testing on platforms other than Linux
* Needs testing with packaging and installation on all platforms
* I'd like to make a few more changes, like creating a "data" directory and moving the .ttf and GeoIP files there.
* Maybe create a "scripts" directory for the Linux launch scripts
* Maybe do the same for the generated docs (make docs)

See also #5457 
That's how I started it but it had the problem that make was seeing the directory having a newer modification time than the other targets inside it so it would always rebuilt everything, even when not needed. @obrakmann suggested this nice workaround.