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.

src/msw/utilsgui.cpp

@@ -138,13 +138,6 @@ bool wxColourDisplay()
     return s_isColour != 0;
 }
 
-// Returns depth of screen
-int wxDisplayDepth()
-{
-    ScreenHDC dc;
-    return GetDeviceCaps(dc, PLANES) * GetDeviceCaps(dc, BITSPIXEL);
-}
-
 void wxDisplaySizeMM(int *width, int *height)
 {
     ScreenHDC dc;