SelectAll() can be slow due to virtual paradigm

[joachimmarder]

SelectAll() can be slow due to virtual paradigm. SelectAll() iterates the tree, which initializes the nodes and so can lead to many calls to the OnInitChildren event handler. Surrounding the call to SelectAll()with BeginUpdate()/ EndUpdate() improves the situation.

Proposed solution: SelectAll() should call BeginUpdate()/ EndUpdate() automatically if an OnInitChildren event handler is assigned.

[joachimmarder]

Not only an OnInitChildren can slow down SelectAll() but other event handlers too. So I removed this condition.