An extension interface allowing an EventBehaviour to Lock the tool handler.
Certain types of events need to be able to "lock" the tool handler
so that it is the only event getting events until it "unlocks" the tool handler.
Example:
The Polygon tool has a BoxSelection behaviour that accepts drag events and draws a
rectangle until the mouse is released (at that point it selects the vertices that are with
the box).
A Composite Mode (See GOF Composite Pattern) where contained modes are ordered by priority and only the
mode with the highest priority and is valid for the current context is ran.
A Composite Mode (See GOF Composite Pattern) where contained modes are ordered by priority and only the
mode with the highest priority and is valid for the current context is ran.