“CalVer” versioning scheme

[fitojb]

In issue #339, @panos said (emphasis mine):

Replace "Tenacity 3.0.4-alpha" with simply "Tenacity", we can figure out the versioning stuff later.

I would like to propose the maintainers to adopt some version of the Calendar Versioning scheme for version numbers. What I like about it is that it completely removes fruitless discussions about marketing-related arbitrary version number bumps from the picture (we’ve been bitten by this in the @LibreOffice project).