CheckBox state propagation slow for large number of nodes

[GoranDespalatovic]

Steps to reproduce :

1. Create grid with 5 nodes each having 250000 child nodes, set all node's checkType to triStateCheckBox.
2. Check root node, grid will select all nodes just fine
3. Expand any of those 5 nodes, deselect checkBox. Grid takes much more time to set check states.

Suggestion :

change following methods

procedure TBaseVirtualTree.WMKeyUp(var Message: TWMKeyUp);
procedure TBaseVirtualTree.HandleMouseUp(var Message: TWMMouse; const HitInfo: THitInfo);

replace

DoCheckClick(FCheckNode, FPendingCheckState);

with

            BeginUpdate;
            try
              DoCheckClick(FCheckNode, FPendingCheckState);
            finally
              EndUpdate;
            end;

I'm not shure if it will break something because of issue #805 but it's worth considering. After code change grid selects checkboxes allmost immediately even on million child nodes.

[joachimmarder]

The BeginUpdate() / EndUpdate() would fit better in ChangeCheckState(), which is called from DoCheckClick(). It should only be called if FCheckPropagationCount = 0. That way any code path to DoCheckClick() benefits.