Added new attributes to Activity

Activity now has fields name, cmdline and exe that will help identify it.
Activities stores Activity to dict

Activity object can represent same activity while have different content (e.g. pid).
I used compound primary key of name and title in dictionary key.
TimeTracker now stores Activity, not just str

TimeTracker has been saving only str with title of Activity to ActivityRecords.
Now it saves whole Activity object.
Waktu-gui now shows Activity.key
Activity info now shows in category tab
Activities.__getitem__ for easy access
Merge branch 'feature/activity-info' of github.com:Artimi/waktu into feature/activity-info

Conflicts:
	tests/waktu_tests.py