Fixed a bug where breakpoint scanning would never finish if the scan
was performed before any other replay actions had happened.
Extract common scanning events and getters into TimelapseScanner.js. Closes #157.
Make _showMessagePanel parameterized by message, or use a
default. Closes #158.
Move TimelapseModel.breakpointScanner into
TimelapseModel.scanners.breakpoint. This allows us to be more
extensible w.r.t. installed scanners later.