Content data is not validated when used as a library

[javagl]

When the validator is used as a library (and not as a command line application), then the content data is currently not validated.

This is caused by the registration of the content data validators only happening in main.ts.

This is already being addressed, and will be fixed with the next release.

[javagl]

This should be fixed in the 0.2.0 release, via #247

The current approach is to call the registration functions whenever a public method is called (and keeping track of whether the registrations have already been performed, using a boolean flag). A conceptually cleaner and more elegant solution could be to use Static Initializer Blocks. But there, some details of the behavior apparently depend on the order in which classes are loaded, which appears to be brittle and error-prone (also see #224 (comment) ).

The approach for registering content types or extension validators may have to be reviewed when there should be an API-level (or even CLI-level) mechanism for defining and supplying custom validators. But until then, the current solution should be a reasonable one.