Sesssion Timeout: initial rough port of 3.1 plugin

1. Exposes plugin keepalive and inactivity checks via the `keepalive.wb-session-timeout` and `inactivity.wb-session-timeout` events.
1. Some functions (e.g. parseTime and getCurrentTime) will be moved to helpers.js once @masterbee lands his change that allows vapour to be extended.
1. Demo text needs updating.