GlslEditor: Some editor creation related cleanup

Use a BaseEditorFactory derived class, move some code around.

Change-Id: Id560a215102016cdbe21809f97be8fc190ed5cf5
Reviewed-by: Christian Stenger <christian.stenger@digia.com>

src/plugins/glsleditor/glslhighlighter.cpp

@@ -92,7 +92,7 @@ void Highlighter::highlightBlock(const QString &text)
     lex.setState(state);
     lex.setScanKeywords(false);
     lex.setScanComments(true);
-    const int variant = GlslEditorWidget::languageVariant(parent()
+    const int variant = languageVariant(parent()
                          ? static_cast<BaseTextDocument*>(parent())->mimeType()
                          : QString());
     lex.setVariant(variant);