Add DoShowCellEditControl() showing the editor unconditionally

It doesn't make sense to perform the checks in ShowCellEditControl()
when it's called from EnableCellEditControl() and this makes the code
unnecessarily fragile as m_cellEditCtrlEnabled needs to be set at just
the right moment for it to work correctly.

Call the new DoShowCellEditControl() instead and perform the checks only
in the public function, for compatibility.

Also note in a comment and the documentation that ShowCellEditControl()
is not very useful anyhow and that EnableCellEditControl() should most
often be used instead.

No real changes (the commit is best viewed ignoring whitespace changes).

interface/wx/grid.h

@@ -3827,8 +3827,9 @@ public:
         Displays the active in-place cell edit control for the current cell
         after it was hidden.
 
-        Note that this method does @em not start editing the cell, this is only
-        done by EnableCellEditControl().
+        This method should only be called after calling HideCellEditControl(),
+        to start editing the current grid cell use EnableCellEditControl()
+        instead.
     */
     void ShowCellEditControl();