Add live cursor-coordinate readout to the element editor status bar (#580)

Displays the cursor's scene position (same grid units as the parts'
X/Y property spinboxes) in a permanent status bar label, updated on
every mouse move. Addresses the overlapping-node mis-click case from
the originating forum report: with a live readout, precise pointing
no longer requires guessing against nearby z-ordered points.

ElementScene::mouseMoveEvent already computed the (optionally
grid-snapped) scene position on every move; it now also emits it via
a new mouseMoved(QPointF) signal, which QETElementEditor's status bar
label subscribes to.

sources/editor/ui/qetelementeditor.h

@@ -161,6 +161,8 @@ class QETElementEditor : public QMainWindow
 
 		QLabel *m_default_informations = nullptr;
 
+		QLabel *m_position_label = nullptr;
+
 };
 
 #endif // QETELEMENTEDITOR_H