[#94] Store deltas inseconds instead of parsing to minutes

Created by: ghost

Activity delta (duration) are received from dbus in a seconds format, instead to parsing them to minutes then compiling them to make the today-summary. We keep them in seconds all the time for more precision.

The time formatting functions have also been rewrite to use seconds instead minutes and to have a cleaner approach using Date() objects and round().

Closes: #94 (closed)