Replace virtual isModified method with a getter/setter/notifier

We have lacked the setter and dedicated notifier before.

Change-Id: I58845a48259d260c5cc90ae94b173c79cddcfef9
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/android/androidmanifestdocument.h

@@ -39,7 +39,6 @@ public:
     bool save(QString *errorString, const QString &fileName = QString(),
               bool autoSave = false) override;
 
-    bool isModified() const override;
     bool isSaveAsAllowed() const override;
 
 private: