Removed CurrentSessionPlayedTime from TaskItem. The "current" concept does not make sense.
The totalplayedtime of a task now it is only calculated when the TaskItem.TotalPlayedTime property is got. We avoid calculating it on every timer tick (it is not necessary).
Now a times summary window can be opened
Added the Open times summary button, command, tests. The task list to be summarized is passed through a message.
Added a PlayedTimesSummaryView and Model.

Added tests for summarizing a task list on that View.
Fixed testing project for NUnit debugging.