OF-546: Move Maven POM files into source directory.
I continue to be thoroughly confused by having two directories per plugin, one for the plugin sources, and one for the maven build. This commit merges both directories.
As a side effect, various plugin unit tests are now executed by default. Some failed, for which I applied fixes.
This commit also adds some plugins to the Maven structure that previously were not.
OF-546: Add metadata to pom.xml
This commit copies version number, author and description from plugin.xml to pom.xml. This makes the same information available as Maven metadata.
When Ant support is dropped, the duplicate data in the plugin.xml should be replaced by a filter (which takes the data from the pom.xml).
