improve window_watcher event structure

[seanbreckenridge]

while parsing, maintain a 'recent' cache, removing items when they haven't had an event for 2 hours

each item would look like:

# represents one history entry
class Entry(NamedTuple):
    times: Tuple[datetime, timedelta]
    application: str
    window_title: str

add some @property wrappers to serialize the start/end times