Add timeline.running property

Add a new “running” property as a companion to “undoing”, that is
true whenever (forward or backward) timeline actions are being
executed.

This will eliminate the need for tricks like the triggered_move
flag in https://github.com/robinhouston/talkie-leaflet/commit/7b83f10d3bab174e7d83df3bbdafe645e7eeed82
Add an onPause method to timeline

Having onPlay but no corresponding onPause can be awkward, as
seen in https://github.com/robinhouston/talkie-leaflet/commit/7b83f10d3bab174e7d83df3bbdafe645e7eeed82
Bump version number

Since the API has changed we’ll bump to 1.3.
Bring wiki up-to-date