Add wxDisplay::GetDepth() and use it for wxDisplayDepth()

Allow getting the depth of any display, not just the primary one, even
though this is not implemented for Unix ports currently.

Mostly do this for consistency with the other display-related functions.

include/wx/private/display.h

@@ -78,6 +78,9 @@ public:
     // return the area of the display available for normal windows
     virtual wxRect GetClientArea() const { return GetGeometry(); }
 
+    // return the depth or 0 if unknown
+    virtual int GetDepth() const = 0;
+
     // return the name (may be empty)
     virtual wxString GetName() const { return wxString(); }