Revert "Replace virtual isModified method with a getter/setter/notifier"

This doesn't fix the issue with cleaning the clear state of undo stack.
This introduced the issue when ui file is opened it's marked as modified.
It reverts 59c90e00c1
and d0c537ca75

Change-Id: Ifd4ff8483d6c297461632de500a4502b1fd0871f
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/texteditor/textdocument.h

@@ -103,6 +103,7 @@ public:
     bool setContents(const QByteArray &contents) override;
     bool shouldAutoSave() const override;
     bool isFileReadOnly() const override;
+    bool isModified() const override;
     bool isSaveAsAllowed() const override;
     void checkPermissions() override;
     bool reload(QString *errorString, ReloadFlag flag, ChangeType type) override;