Debugger: Streamline watch editing code.

Implement WatchModel::data() to return the right values
for Qt::EditRole, obsoleting the role matching in the delegate.
Similarly, handle Qt::EditRole in setData(), leaving only
'Change watch expression' in the delegate, which removes
and recreates a row, causing crashes when done in setData().

src/plugins/debugger/watchhandler.h

@@ -119,7 +119,7 @@ signals:
     void enableUpdates(bool);
 
 private:
-    QByteArray niceType(const QByteArray &typeIn) const;
+    QString displayType(const WatchData &typeIn) const;
     void formatRequests(QByteArray *out, const WatchItem *item) const;
     DebuggerEngine *engine() const;
     int itemFormat(const WatchData &data) const;