Inherit art provider in wxAuiFloatingFrame by default

Use the same art provider for a floating frame detached from an existing
wxAuiManager as was used by the original wxAuiManager itself, to ensure
that the appearance of this frame is consistent with the appearance of
its parent.

Implementing this required adding wxAuiDockArt::Clone() to allow copying
it in the new frame and this patch also adds GetAuiManager() to
wxAuiFloatingFrame, similar to the existing method in wxAuiNotebook, in
order to allow changing the dock art from the application code if
desired.

Closes https://github.com/wxWidgets/wxWidgets/pull/2022

Closes #18882.

include/wx/aui/dockart.h

@@ -35,6 +35,7 @@ public:
     wxAuiDockArt() { }
     virtual ~wxAuiDockArt() { }
 
+    virtual wxAuiDockArt* Clone() = 0;
     virtual int GetMetric(int id) = 0;
     virtual void SetMetric(int id, int newVal) = 0;
     virtual void SetFont(int id, const wxFont& font) = 0;
@@ -92,6 +93,7 @@ public:
 
     wxAuiDefaultDockArt();
 
+    wxAuiDockArt* Clone() wxOVERRIDE;
     int GetMetric(int metricId) wxOVERRIDE;
     void SetMetric(int metricId, int newVal) wxOVERRIDE;
     wxColour GetColour(int id) wxOVERRIDE;