Implement a better Conan support, especially on macOS

What does this MR do?

This MR:

• Remove libxml2 and zlib dependencies on macOS
• Use DYLD_FALLBACK_LIBRARY_PATH instead of LD_LIBRARY_PATH because it is unsupported in macOS and DYLD_LIBRARY_PATH prevents using ANY system libs.
• Remove unneeded activities bundle dependency to gui
• Fix application script template generation to use : as path separator instead of ;
• Cleanup template script
• Add VLC conan package for Windows and macOS, use system for Linux
• Add scripts to launch unit tests and utilities with conan packages
• bonus: fix ioAtoms and ioITK unit tests (updated object access type)

How to test it?

• Enable USE_CONAN in CMake configuration to build Sight
• Build Sight
• Run some app/UT/utilities with associated shell script (available in bin folder in your build_dir)

Data

No data required

Some results

Sight build with conan package

Additional tests to run

• Build on Windows
• Build on Linux (Mint 18)
• Build on Macos (package available for xcode 9.1 and 10.0 🎉 )

Related MR

Closes #171 (closed)