IMetaclassSelector is a reusable component wrapping an SWT Text widget that can
be used to select a single metaclass.
The proposal menu is popped whenever one of the following conditions is
fulfilled:
the user enters any upper case letter character (as metaclass names
always starts with a Capital letter)
the user enters CTRL+SPACE which is the standard completion activation
sequence
The metaclasses proposed for completion are those defined in the metamodel
filtered by
IMetaclassSelector.setMetaclassFilter(Predicate) if such a filter is set.