Lay out various wxPG dialogs based on the current system screen design.

Currently, customizing some dialogs for small screen environment is done statically on the build stage by setting wxPG_SMALL_SCREEN to 1 (what is done for WXWINCE only).
Instead, we can do this for all platforms in a dynamic manner, based on the classification provided by wxSystemSettings::GetScreenType() function.
In order to do so there is introduced a static helper function wxPropertyGrid::IsSmallScreen() which is used to select right parameters (size, position) for several dialogs in order to display them correctly on the small screen.

include/wx/propgrid/propgrid.h

@@ -1556,6 +1556,12 @@ public:
     static wxString& CreateEscapeSequences( wxString& dst_str,
                                             wxString& src_str );
 
+    // Checks system screen design used for laying out various dialogs.
+    static bool IsSmallScreen()
+    {
+        return wxSystemSettings::GetScreenType() <= wxSYS_SCREEN_PDA;
+    }
+
     /**
         Returns rectangle that fully contains properties between and including
         p1 and p2. Rectangle is in virtual scrolled window coordinates.