Consider layers in 'sieben-manage dot'

[ahitrin]

Currently, sieben-manage dot exports a whole goal tree from the given file. It's not useful in many situations.

To make its output closer to what user may see in Sieben UI, we should add an opportunity to consider all existing layers and optionally enable/disable existing views.

E.g.:

# by default, shows only open goals (or selected)
sieben-manage dot my.db
# show both open and closed goals (or selected)
sieben-manage dot -n my.db
# show both open and closed goals, but only switchable (or selected)
sieben-manage dot -nt my.db
# show only open (or selected) goals with progress counters
sieben-manage dot -p my.db

Feature drop: this improvement will make sieben-manage unable to mitigate zoom roots. That's OK for now. There is a workaround: to export a full goal tree we could unzoom it in Sieben UI, perform an export, and zoom it back.