Fix line draw mode can cause undo/redo to de-sync

Previously placed lines connected to one currently being drawn are
not properly undone de-syncing scene and undo/redo mechanism. The
solution is to abort any line insertions before the actual undo.

To achieve this action_stack_model.py was extended to emit aboutToUndo
and aboutToRedo signals before undoing and redoing. These are used by
edit_schematic_view.py to abort the line insertion.

Fixes #20