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/dialogs/codecselector.cpp

@@ -87,6 +87,8 @@ CodecSelector::CodecSelector(QWidget *parent, Core::BaseTextDocument *doc)
     int currentIndex = -1;
     foreach (int mib, sortedMibs) {
         QTextCodec *c = QTextCodec::codecForMib(mib);
+        if (!doc->supportsCodec(c))
+            continue;
         if (!buf.isEmpty()) {
 
             // slow, should use a feature from QTextCodec or QTextDecoder (but those are broken currently)

src/plugins/coreplugin/textdocument.cpp

@@ -183,7 +183,13 @@ void BaseTextDocument::setCodec(const QTextCodec *codec)
 {
     if (debug)
         qDebug() << Q_FUNC_INFO << this << (codec ? codec->name() : QByteArray());
-    d->m_format.codec = codec;
+    if (supportsCodec(codec))
+        d->m_format.codec = codec;
+}
+
+bool BaseTextDocument::supportsCodec(const QTextCodec *) const
+{
+    return true;
 }
 
 void BaseTextDocument::switchUtf8Bom()

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;