Frontend: split the listing of specific savepoints
(SavepointListProvider) from savepoint creation
(ReplaySavepointTracker).

Closes #164.
Frontend: spruce up many icons in the status bar, and add a mini-icon
to the scans menu.
Frontend: add error message to assert inside Object.add/removeEventListener().
Frontend: Rewrite event listener for TimelapseControllerView status
bar buttons. Make all said event listeners go through the
_modifyListeners() pattern.

Add a helper to WebInspector.View which caches bound listeners for use
with DOM event listeners (which do not take an explicit this object).

Fix a name conflict in TimelapseOverview that conflicted with the new
View helper method's data structures.
Frontend: split off different savepoint restore algorithms into different classes.

Closes #215.