Added the concept of 'sticky' to tools.

When you select either the AddZone tool, or the AddCombatant tool they
now default to a mode where after you click once, they default back to
the Pan tool. However, if you click them a _second time_ they visualy
change, and are now 'stickied'. They will continue to add zones or
combatants until you click them again, or change tools. This gives us
the best of both worlds.
Merge pull request #57 from lordnull/multiadd_combatants

Added the concept of 'sticky' to tools.