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.

interface/wx/gdicmn.h

@@ -1191,6 +1191,10 @@ bool wxColourDisplay();
     Returns the depth of the display (a value of 1 denotes a monochrome
     display).
 
+    @note Use of this function is not recommended in the new code as it only
+        works for the primary display. Use wxDisplay::GetDepth() to retrieve
+        the depth of the appropriate display instead.
+
     @header{wx/gdicmn.h}
 */
 int wxDisplayDepth();