Validate correspondence between FXML and Java

When using field injection, there needs to be a correspondence between both the field name and type and FXML fx:id and tag (type). Both these should be validated, e.g. to catch errors where the java code imports java.awt.Label instead of the JavaFX Label class. Validating is one thing, another where to indicate the error. If SB sees an fx:id without a corresponding Java id, it could be indicated as a problem in SB, e.g. a red border around the fx:id field, or it could be a problem on the controller class. If Java has an @FXML annotated field with no corresponding fx:id or the type doesn't match, it should (also) be marked as a Java problem. The validation could be limited to open FXML and Java files, to make the validation less heavy-weight.

A similar validation should happen for event handlers, i.e. onXyz="#method" attributes.

With markers in the Java file, we should have Quick Fixes for adding/fixing fields and methods, which may be more natural than a one-shot "generate controller fields and method action" (see #7 (closed)).