Updates code to use sdt.core methods

The methods of sdt.editor have been moved in sdt.core
Modifies the way to get an editor from a viewer

The editor is stored in the TextViewer using getData/setData.
Remove the sdt.editor code
Adapts to completion proposal change
Fixes the compilation problem, but breaks Scala hovers in play template

editors
Merge pull request #225 from skyluc/issue/API-move-delete-editor

Removes the sdt.editor plugin