[Widgets] register transparency property of Gui::ColorButton

- at the moment it is not possible to set the property "allowTransparency" in Qt Designer

src/Tools/plugins/widget/customwidgets.cpp

@@ -1762,7 +1762,10 @@ void PrefDoubleSpinBox::setParamGrpPath ( const QByteArray& name )
 // -------------------------------------------------------------
 
 ColorButton::ColorButton(QWidget* parent)
-    : QPushButton( parent ), _allowChange(true), _drawFrame(true)
+    : QPushButton(parent),
+      _allowChange(true),
+      _drawFrame(true),
+      _allowTransparency(false)
 {
     _col = palette().color(QPalette::Active,QPalette::Midlight);
     connect( this, SIGNAL( clicked() ), SLOT( onChooseColor() ));
@@ -1793,6 +1796,16 @@ bool ColorButton::allowChangeColor() const
     return _allowChange;
 }
 
+void ColorButton::setAllowTransparency(bool ok)
+{
+    _allowTransparency = ok;
+}
+
+bool ColorButton::allowTransparency() const
+{
+    return _allowTransparency;
+}
+
 void ColorButton::setDrawFrame(bool ok)
 {
     _drawFrame = ok;