Add support for freezing columns and/or rows of wxGrid

Add wxGrid::FreezeTo() method which allows to freeze the given number of
columns and/or rows at the beginning of the grid, i.e. keep them pinned
in place while the rest of the grid is scrolled.

The main wxGridWindow (m_gridWin) now corresponds to the non-frozen part
of the grid, with up to 3 new similar windows for the frozen
rows/columns and the frozen corner cells (which only exist if both rows
and columns are frozen) being additionally used.

Doing this involved adding "wxGridWindow*" parameter to many functions
that previously only worked with m_gridWin itself and addressing
additional complications, such as mouse events that can now cross
different windows.

See https://github.com/wxWidgets/wxWidgets/pull/952 for the original
version of the changes.

samples/grid/griddemo.h

@@ -75,6 +75,8 @@ class GridFrame : public wxFrame
     void SelectCols( wxCommandEvent& );
     void SelectRowsOrCols( wxCommandEvent& );
 
+    void FreezeOrThaw( wxCommandEvent& );
+
     void DeselectCell(wxCommandEvent& event);
     void DeselectCol(wxCommandEvent& event);
     void DeselectRow(wxCommandEvent& event);
@@ -204,6 +206,8 @@ public:
         ID_SIZE_LABELS_ROW,
         ID_SIZE_GRID,
 
+        ID_FREEZE_OR_THAW,
+
         ID_SET_HIGHLIGHT_WIDTH,
         ID_SET_RO_HIGHLIGHT_WIDTH,