Highlight the grid row/column being dragged on all platforms

Remove __WXGTK__ checks and highlight it everywhere, this can be useful
under other platforms too depending on the colour schema and not having
platform-specific checks is better than having them if they're not
absolutely required.

If we really want to avoid highlighting in some cases, we should check
for the contrast between the border and background colours, but it seems
to be much simpler to just always show it.

src/generic/grid.cpp

@@ -7114,10 +7114,9 @@ void wxGrid::DrawRowLabel( wxDC& dc, int row )
         rend.DrawBorder(*this, dc, rect);
     else
     {
-#ifdef __WXGTK__
+        // just highlight the current row
         dc.SetPen(wxPen(wxSystemSettings::GetColour(wxSYS_COLOUR_HIGHLIGHT)));
         dc.DrawRectangle(rect);
-#endif
         rect.Deflate(GetBorder() == wxBORDER_NONE ? 2 : 1);
     }
 
@@ -7271,10 +7270,9 @@ void wxGrid::DrawColLabel(wxDC& dc, int col)
             rend.DrawBorder(*this, dc, rect);
         else
         {
-#ifdef __WXGTK__
+            // just highlight the current column
             dc.SetPen(wxPen(wxSystemSettings::GetColour(wxSYS_COLOUR_HIGHLIGHT)));
             dc.DrawRectangle(rect);
-#endif
             rect.Deflate(GetBorder() == wxBORDER_NONE ? 2 : 1);
         }
     }