Add support for tasks privacy settings

[raimund-schluessler]

Closes #243.

This adds a dropdown with the options

• When shared show full event
• When shared show only busy
• When shared hide this event

to the details view.

[rkaraba]

if you share task list with can edit privilegies
and you set task privacy to When shared show only busy
there is a problem
When other user changes anything in this task than original title of this task is replaced with text Busy

[raimund-schluessler]

if you share task list with can edit privilegies
and you set task privacy to When shared show only busy
there is a problem
When other user changes anything in this task than original title of this task is replaced width text Busy

Hm, that's a tricky one. Same happens in the Calendar app. I will create a separate issue for this and discuss things there.