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.

src/propgrid/propgrid.cpp

@@ -1696,10 +1696,12 @@ void wxPropertyGrid::SetCurControlBoldFont()
 wxPoint wxPropertyGrid::GetGoodEditorDialogPosition( wxPGProperty* p,
                                                      const wxSize& sz )
 {
-#if wxPG_SMALL_SCREEN
-    // On small-screen devices, always show dialogs with default position and size.
-    return wxDefaultPosition;
-#else
+    if ( IsSmallScreen() )
+    {
+        // On small-screen devices, always show dialogs with default position and size.
+        return wxDefaultPosition;
+    }
+
     int splitterX = GetSplitterPosition();
     int x = splitterX;
     int y = p->GetY();
@@ -1729,7 +1731,6 @@ wxPoint wxPropertyGrid::GetGoodEditorDialogPosition( wxPGProperty* p,
         new_y = y + m_lineHeight;
 
     return wxPoint(new_x,new_y);
-#endif
 }
 
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