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/common/dpycmn.cpp

@@ -122,6 +122,13 @@ wxRect wxDisplay::GetClientArea() const
     return m_impl->GetClientArea();
 }
 
+int wxDisplay::GetDepth() const
+{
+    wxCHECK_MSG( IsOk(), 0, wxT("invalid wxDisplay object") );
+
+    return m_impl->GetDepth();
+}
+
 wxString wxDisplay::GetName() const
 {
     wxCHECK_MSG( IsOk(), wxString(), wxT("invalid wxDisplay object") );