Rephrase javadoc

Clarified the Converter javadoc, from suggestions by ctrueden.
ConvertService: return collection

Instead of returning a List of input candidates, we can generalize to
Collection.
Converter: remove abstract getXXXType methods

Instead of passing Class instances via constructors, concrete objects
should just implement the methods with an appropriate return value, for
consistency across SJC.
Refactor populateInputs method

* Now accepts Collection instead of just Set
* Renamed to populateInputCandidates for clarity
AbstractConverter: rephrase javadoc

Updated javadoc for clarity, as written by ctrueden
WidgetModel: rename converted objects collection

convertedObjectPool > convertedObjects, to simplify and avoid confusion
with "objectPool".
WidgetModel: typo fixes

Fixed typos in the WidgetModel javadoc.