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 
I actually like that it works both for installed and self-compiled version. This helps during development and when people set up a new https://github.com/OpenRA/OpenRA/wiki/Dedicated from an installed package.