Core: introduce codec support check for text documents

So indiviual documents can block codecs they do not support like ui or
qml files.

Change-Id: I65b605762c696d38be9049f0064fd68aad0193da
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/coreplugin/textdocument.h

@@ -46,6 +46,7 @@ public:
     Utils::TextFileFormat format() const;
     const QTextCodec *codec() const;
     void setCodec(const QTextCodec *);
+    virtual bool supportsCodec(const QTextCodec *) const;
     void switchUtf8Bom();
     bool supportsUtf8Bom() const;
     Utils::TextFileFormat::LineTerminationMode lineTerminationMode() const;