Generate and set common build qualifier per project

[berndv]

Instead of using a separate build qualifier for each plug-in, there should be a single qualifier for all plug-ins within a module.

• Maven unique snapshot qualifier for snapshot builds (ensure that qualifier of deployed module artifacts and contained plug-ins is the same)
• Provide qualifier and timestamp properties even for projects with non-qualified versions, e.g. release versions
• Disable Tycho qualifier Mojo -> If done, maybe we can also disable Tycho version validation and get rid of the project.version properties mess (See Ensure that ${project.version} always satisfies the Maven version format  #95)