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/qmljs.qbs

@@ -28,6 +28,7 @@ QtcLibrary {
             "qmljscheck.cpp", "qmljscheck.h",
             "qmljscodeformatter.cpp", "qmljscodeformatter.h",
             "qmljscompletioncontextfinder.cpp", "qmljscompletioncontextfinder.h",
+            "qmljsconstants.h",
             "qmljscontext.cpp", "qmljscontext.h",
             "qmljsdelta.cpp", "qmljsdelta.h",
             "qmljsdocument.cpp", "qmljsdocument.h",