[REFACTOR] moved getShell method from EditorAPI to SWTUtils

I do not really know what an EditorAPI has to with SWT shells. Moved the
method into the right utility class.

Change-Id: I5673f5ed564923ce660a3562dd562411045ecf83
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1212
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>