Selects all sketch objects.
The SelectAll method selects all the items currently in the sketch. Normally, you won't have to call this since the Ctrl+A shortcut is already built into the SRP Sketch Control. However, if you have a frame with a menu item using Ctrl+A, then that menu will override the ability for the SRP Sketch Control to do this automatically. In that case, have your menu accelerator call this method when the SRP Sketch Control has focus.