Fix infinite recursion if wxGrid is modified from event handler

Calling wxGrid::{Insert,Delete}{Rows,Cols}() from wxEVT_GRID_CELL_CHANGE
event handler resulted in infinite recursion because it tried to hide
the grid editor control again, which resulted in another CELL_CHANGE
event being generated and so on.

Break this infinite recursion in the usual way, i.e. by updating the
state of wxGrid before invoking the user-defined event handler.

This required separating SaveEditControlValue() in 2 functions, the main
one retaining IsCellEditControlEnabled() check for compatibility, and
the new DoSaveEditControlValue() that can be called even after disabling
the editor.

Closes #2287.

Closes https://github.com/wxWidgets/wxWidgets/pull/1540

include/wx/generic/grid.h

@@ -2482,6 +2482,10 @@ private:
     void SetNativeHeaderColCount();
     void SetNativeHeaderColOrder();
 
+    // Unlike the public SaveEditControlValue(), this method doesn't check if
+    // the edit control is shown, but just supposes that it is.
+    void DoSaveEditControlValue();
+
     // these sets contain the indices of fixed, i.e. non-resizable
     // interactively, grid rows or columns and are NULL if there are no fixed
     // elements (which is the default)