Collapsed mode conditions

Adds an option to allow the user dictate when DashClock should show
as collapsed based on the number of visible extensions.

ie. Collapse when 2+ extensions are visible would keep the small
widget expanded with only 1 extension visible, but as soon as 2+
were visible it would collapse as normal.

You can also force DashClock to never collapse regardless of available
height space or size of widget.

This also changes DashClock to by default collapse the widget when
using slim DPI.

Example: http://goo.gl/UCPpXh

Change-Id: I22071524053b4b234b39966c7a95c4dded7530d8