qmljs: added qmljsconstants.h

Moved Document::Language, Import::Type and StaticAnalysis::Severity
enums to qmljsconstants.h and renamed values removing the redundant
part.
Thus the effective length changed little or improved
(Document::QmlLanguage => Language::Qml).

The separate file allows better reuse of enum values without introducing
circular dependencies.

Change-Id: I5186d7c04f5d3f6c289068b919be5ff1ff118326
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>

src/libs/qmljs/parser/qmljsparser_p.h

@@ -162,7 +162,7 @@ public:
     inline DiagnosticMessage diagnosticMessage() const
     {
         foreach (const DiagnosticMessage &d, diagnostic_messages) {
-            if (d.kind != DiagnosticMessage::Warning)
+            if (d.kind != Severity::Warning)
                 return d;
         }