Improve Installation Ergonomics on Linux

[ftilde]

Currently there are several limitations for installing voreen system-wide:

• The appimage is built during the installation step (not necessary, obviously)
• With a "standard" CMAKE_INSTALL_PREFIX such as "/usr/local" we create non-standard directories such as resource/doc/... into /usr/local and dump libraries, binaries, license, changelog, ... directly into `/usr/local/
• The voreen binary cannot be moved from its location after installation because it uses argv[0] to locate resources.
• Starting voreenve from $PATH is also not possible due to the argv[0] issue. A possible fix would be to set the basePath_ to CMAKE_INSTALL_PREFIX. There might be other issues with this approach, though (for example when building an appimage).