Reflect unsaved-changes state in the main window title

On macOS in particular there's currently no way to tell from the window
chrome alone whether the active project has unsaved changes. The main
window's title is set once in the constructor to a static string and
never updated afterward, and QET never sets Qt's windowModified
property anywhere -- so the native "document modified" indicator
(the dot in the close button on macOS; an asterisk in the title on
platforms that render it as text) never appears.

Add QETDiagramEditor::updateWindowModifiedState(), which sets the
window title to "<project>[*] - QElectroTech" (the "[*]" is Qt's own
placeholder convention for this) and calls setWindowModified() with
the active project's own modified flag. Call it from two places:

- subWindowActivated(), the single existing choke point already used
  whenever the visible MDI tab changes, so switching projects
  immediately reflects the newly active one's own state.
- A new per-project connection to QETProject::projectModified, added
  in addProject() alongside the existing undo-stack registration,
  filtered to only act when the modified project is the currently
  active one.

With no project open, the title and modified flag both revert to the
original static, unmodified state.

Implements https://github.com/qelectrotech/qelectrotech-source-mirror/discussions/596

sources/qetdiagrameditor.h

@@ -98,6 +98,7 @@ class QETDiagramEditor : public QETMainWindow
 		ProjectView *findProject(const QString &) const;
 		QMdiSubWindow *subWindowForWidget(QWidget *) const;
 		void updateUsageTrackersActiveState();
+		void updateWindowModifiedState();
 
 	signals:
 		void syncElementsPanel();