Add new test file that contains tags.
Add tags property to Headline.
Implement parsing and storing of tags in headlines.
Add Tags class, a visitor to query tags.
Implement first version of findElement.

This is the start of a set of functions that search and filter
OrgElement structures.
Implement tag searching within elements and their parents.
Extend test to verify tag parsing. Works fine.
Extend test for SimpleTree.

Uses the new findElement logic.
Extend the Clock test, now that we have findElement.