Allow disabling hiding columns when using wxHeaderCtrl in wxGrid

Add wxGrid::DisableHidingColumns() method which can be used to prevent
wxHeaderCtrl from allowing the user to hide columns interactively, which
is something it allows to do by default, unlike the "built-in" wxGrid
header.

Also add EnableHidingColumns() and CanHideColumns() for consistency with
the other similar methods.

Closes https://github.com/wxWidgets/wxWidgets/pull/1554

src/generic/grid.cpp

@@ -2607,6 +2607,7 @@ void wxGrid::Init()
     m_gridFrozenBorderPenWidth = 2;
 
     m_canDragColMove = false;
+    m_canHideColumns = true;
 
     m_cursorMode  = WXGRID_CURSOR_SELECT_CELL;
     m_winCapture = NULL;
@@ -4829,6 +4830,18 @@ bool wxGrid::EnableDragColMove( bool enable )
     return true;
 }
 
+bool wxGrid::EnableHidingColumns(bool enable)
+{
+    if ( m_canHideColumns == enable || !m_useNativeHeader )
+        return false;
+
+    GetGridColHeader()->ToggleWindowStyle(wxHD_ALLOW_HIDE);
+
+    m_canHideColumns = enable;
+
+    return true;
+}
+
 void wxGrid::InitializeFrozenWindows()
 {
     // frozen row windows