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/display.h

@@ -74,6 +74,9 @@ public:
     // get the client area of the display, i.e. without taskbars and such
     wxRect GetClientArea() const;
 
+    // get the depth, i.e. number of bits per pixel (0 if unknown)
+    int GetDepth() const;
+
     // name may be empty
     wxString GetName() const;