Always use center point of map events for checking distance
Add reachable_from key to events

Closes #85

reachable_from is a rectangle of tiles where the hero can be standing to
be able to click on an event (provided they are within INTERACT_RANGE)
Reset actionbar slot state when triggering a transform power

Fixes #840
Merge pull request #841 from dorkster/event_facing

Add reachable_from key to events
Move actionbar slot resetting to its own function
Merge pull request #842 from dorkster/bugfixes

Reset actionbar slot state when triggering a transform power