[rframe] make all RFrame constructors private

RFrame can only be created via RPadBase methods.
Idea that each drawable can decide if it requires frame for drawing or
not. 
 

graf2d/gpadv7/inc/ROOT/RFrame.hxx

@@ -33,7 +33,8 @@ namespace Experimental {
 */
 
 class RFrame : public RDrawable  {
-public:
+
+   friend class RPadBase;
 
 private:
    RAttrMargins fMargins{this, "margin_"};     ///<!
@@ -49,7 +50,6 @@ private:
    /// Palette used to visualize user coordinates.
    RPalette fPalette;
 
-public:
    // Default constructor
    RFrame() : RDrawable("frame")
    {
@@ -59,6 +59,11 @@ public:
    /// Constructor taking user coordinate system, position and extent.
    explicit RFrame(std::vector<std::unique_ptr<RPadUserAxisBase>> &&coords);
 
+   RFrame(const RFrame &) = delete;
+   RFrame &operator=(const RFrame &) = delete;
+
+public:
+
    const RAttrMargins &GetMargins() const { return fMargins; }
    RFrame &SetMargins(const RAttrMargins &margins) { fMargins = margins; return *this; }
    RAttrMargins &Margins() { return fMargins; }

graf2d/gpadv7/src/RPadBase.cxx

@@ -174,7 +174,7 @@ std::shared_ptr<ROOT::Experimental::RFrame> ROOT::Experimental::RPadBase::GetOrC
 {
    auto frame = GetFrame();
    if (!frame) {
-      frame = std::make_shared<RFrame>();
+      frame.reset(new RFrame());
       fPrimitives.emplace_back(frame);
    }