#431 Recognizers often ascribe wrong file type

[tfesenko]

Fix contentType for an empty YAML file, now it returns a more generic
YEdit editor.
Why this fix works

1. An empty file always returns INDETERMINATE as description. Note
   that it's not the VALID which is returned when a content describer
   actually accepts the input. See
   com.reprezen.swagedit.core.editor.TextContentDescriber.describe(InputStream, IContentDescription):

        String content = CharStreams.toString(new
InputStreamReader(contents));
        if (content.trim().isEmpty()) {
            return INDETERMINATE;
        }

        return isSupported(content) ? VALID : INVALID;

2. All ContentDescriber for a given file extensions are equally
   important.
   Because all the ContentDescriber return INDETERMINATE, they are
   treated equally important, see
   org.eclipse.core.internal.content.ContentTypeCatalog.collectMatchingByContents(int, IContentType[], List<ContentType>, ILazySource, Map<String, Object>)

3. Then IContentType are sorted and the first one wins
   org.eclipse.core.internal.content.ContentTypeCatalog.internalFindContentTypesFor(ILazySource, IContentType[][], Comparator<IContentType>, Comparator<IContentType>)
   sorts the inputs. In case of INDETERMINATE,
   ContentTypeCatalog.policyConstantGeneralIsBetter is used which is...

A sorting policy where the more generic content type wins.
Lexicographical comparison is done as a last resort when all other
criteria fail.

where more generic content type means, getDepth() is based on the
hierarchy defined by the base-type:

// first criteria: depth - the lower, the better
int depthCriteria = type1.getDepth() - type2.getDepth();

---

By using YEdit as the base-type, we make the YEdit the more generic
content type => it will be used in case of empty files.

[ghillairet]

Passed QA