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/elementscene.h

@@ -179,6 +179,8 @@ class ElementScene : public QGraphicsScene
 		void elementInfoChanged();
 		/// Signal emitted when the element type changes
 		void elementTypeChanged();
+		/// Signal emitted with the current cursor position (scene coordinates) on every mouse move
+		void mouseMoved(const QPointF &pos);
 };
 
 Q_DECLARE_OPERATORS_FOR_FLAGS(ElementScene::ItemOptions)