UiBinder templates code completion problems.

There are many problems with code completion for UI.xml files.
1. The most often one - only few properties are shown in ctrl + space dropdown instead of full list.
2. For enums values - no suggestions at all.
3. It would be nice to show javadoc comments for properties in dropdown list, it would improve development process a lot.

Start by reproducing Ctrl+Space completion in UI.xml UiBinder templates and inspect the plugin entry point that supplies the dropdown suggestions. Done means properties show their full list, enum values receive suggestions, and property Javadoc comments appear in the dropdown; no specific source file or test is named in the issue.

索引モデルが issue の本文から書いたものです。