Save POSIX timestamp instead of formatted string

Previously, the user could break all timestamps of his entire history
file just by switching the format string, which we can not allow to
happen. Now the format is only used when migrating the data and
displaying the absolute timestamp in the panel.

Closes #28.
Add commands to palette for accessing settings

I did this with all my other packages and I hope other package authors
do it too. Using the menu compared to the fuzzy search of the palette is
just ridiculous.
Match default timestamp format in settings

They were different which could result in ... issues before the switch
to POSIX timestamps. Anyway, they shouldn't be different in the first
place so the code's now matches the settings'.

See #28.
Revert c6976fc2..d06d1256 (1.6.1 release prep)

We're not ready for release and with the latest changes it must be a new
minor anyway.
Shorten clean_history a bit