This involves adding a new opcode, event_whenstageclicked, and adding a
method to the blocks container to update the opcode pair depending on
whether the target is the stage. This method is then called in two
places: first after the sb2 import parses the blocks (not done in the
sb2 parsing itself because no other blocks are target specific) and then
again when blocks are shared between targets.
Also added tests for the block container method, and a fixture project
that tests the opcode splitting on sb2 import.
