Fix #292900: Expose Element.pos X/Y values to plugins.

Expose Element.pos X/Y values so that plugin's can accurately
position things like STAFF_TEXT on the score. One example is
to position a tin whistle finger hole pattern directly under
a grace note. Another is to place note names properly using
MuseScore's shipped notename.qml plugin.

mscore/plugin/api/elements.h

@@ -73,6 +73,11 @@ extern Element* wrap(Ms::Element* se, Ownership own = Ownership::SCORE);
 class Element : public Ms::PluginAPI::ScoreElement {
       Q_OBJECT
 
+      /**
+       * Parent element for this element.
+       * \since 3.3
+       */
+      Q_PROPERTY(Ms::PluginAPI::Element* parent READ parent)
       /**
        * X-axis offset from a reference position in spatium units.
        * \see Element::offset
@@ -84,10 +89,29 @@ class Element : public Ms::PluginAPI::ScoreElement {
        */
       Q_PROPERTY(qreal offsetY READ offsetY WRITE setOffsetY)
       /**
-       * Parent element for this element.
-       * \since 3.3
+       * Reference position of this element relative to its parent element.
+       *
+       * This is an offset from the parent object that is determined by the 
+       * autoplace feature. It includes any other offsets applied to the 
+       * element. You can use this value to accurately position other elements 
+       * related to the same parent.
+       *
+       * This value is in spatium units for compatiblity with Element.offsetX.
+       * \since MuseScore 3.3
        */
-      Q_PROPERTY(Ms::PluginAPI::Element* parent READ parent)
+      Q_PROPERTY(qreal posX READ posX)
+      /**
+       * Reference position of this element relative to its parent element.
+       *
+       * This is an offset from the parent object that is determined by the 
+       * autoplace feature. It includes any other offsets applied to the 
+       * element. You can use this value to accurately position other elements 
+       * related to the same parent.
+       *
+       * This value is in spatium units for compatiblity with Element.offsetY.
+       * \since MuseScore 3.3
+       */
+      Q_PROPERTY(qreal posY READ posY)
 
       API_PROPERTY( subtype,                 SUBTYPE                   )
       API_PROPERTY_READ_ONLY_T( bool, selected, SELECTED               )
@@ -326,6 +350,9 @@ class Element : public Ms::PluginAPI::ScoreElement {
       void setOffsetX(qreal offX);
       void setOffsetY(qreal offY);
 
+      qreal posX() const { return element()->pos().x() / element()->spatium(); }
+      qreal posY() const { return element()->pos().y() / element()->spatium(); }
+
       Ms::PluginAPI::Element* parent() const { return wrap(element()->parent()); }
 
    public: