Support POM-less projects, based on Tycho's heuristics

[hallvard]

The POM-less Eclipse projects that Tycho now supports (see http://blog.vogella.com/2015/12/15/pom-less-tycho-builds-for-structured-environments/) are not recognised by version-tiger, so I guess their MANIFEST.MF files will need to be updated by hand. It shouldn't be too difficult to

1. recognise child projects without POM (i.e. children of projects with POM) that look like Eclipse plugin, test, feature and repo projects and follow Tycho's naming conventions
2. make re-versioning behave as if they had a POM, i.e. update the MANIFEST.MF according to the parent's (new) version.
   It does not make sense to be able to select (or de-select) such POM-less projects independently from their parents (with pom), but the user interface could indicate they are considered and will be update, e.g. by auto-(de)selecting them with their parent.

[hallvard]

I took a quick look at the code, and it seems pretty straight-forward. You need to detect that a project is indeed a POM-less variant of a know project type. Then you can create "fake" POM-content to satisfy the code that use the getPomContent() method. Second, return null from getPomXmlFile() and add logic in MavenProjectImpl.setProjectVersionInPom that checks for this null value and avoid writing the pom back.

[hallvard]

I now have a working version, based on the ideas above. Here's a brief summary of the changes:

• A new AbstractEclipseProject includes a reference to a MavenProject, which when set indicates that this is a POM-less Eclipse project (plugin or feature).
• The ProjectFactory is extended to recognise POM-less projects using Tycho's heuristics.
• The pom.xml content (String) is generated from the information in Eclipse's meta-file (MANIFEST.MF and feature.xml), the appropriate packaging (eclipse-plugin, eclipse-test-plugin or eclipse-feature) and the parent pom.xml.
• The Manifest and ManifestSection classes have new methods for accessing header data, to get the Bundle-SymbolicName.
• MavenProjectImpl provides protected access to pomContent, to allow synthesising it.
• MavenProjectImpl and ReferenceUpdater must allow updating project that inherit their version, since the MANIFEST of POM-less Eclipse projects need to be updated.
• MavenProjectImpl must avoid writing to a pom.xml that doesn't exist.
  If this sounds OK, I'll prepare a PR, so you can look at the details. Note, I haven't updated the UI, you must ensure to select or de-select POM-less projects with their parent.