The next CamiTK version should be called CamiTK 5.0

Current CamiTK develop version (aka version 4.2.develop) should become CamiTK version 5.0

• main reason 1: the new viewer extension breaks the API (viewers are not singleton, they have to be accessed by name not pointer, cf #110 (closed))
• main reason 2: viewer extension deserves a major version update to stand out
• The next CamiTK should be very stable as we don't plan to add any big features for a good while, it can be nicknamed as something like "LTS", which deserves a round version number
• A round version number will simplify discussion and communication (latest = stable = lts = 5.0) which stands out much more that 4.2 (very close to 4.1)
• the website explanation can be simplified if the next version is 5.0
• The next color codename is green! 😄

Acceptance tests

• all 5.0 stuff should be renamed 6.0 (deprecation should be move up one version to let people get used to it)
• all 4.2 stuff should be 5.0 (not 4.2 string)
• experimental website should be simplified to point to 5.0 as the new stable / LTS version