Remove an assert when switching a tool while doing a stroke

Well, using macro for this purpose might be a bit ugly, but this is the
best thing we can do until we invent some strategy-based event handling
for the tools.

Ideally, every tool should provide a set of strategies for each of the
action. Such strategy could be registered in the tool proxy on
KisTool::activate() and deregistered on KisTool::deactivate(). But we
are on a long way till that.