Make task manager apply slim margins if defined by default and extend to borders

This patch uses the new appletinterface property to extend to borders. To have consistently sized icons in a panel, the task manager now also uses the slimMargin panel property (when defined) to define the margin of the icons contained. This way they will be consistent with other applets using that margin.

See also:

!92 (merged)

frameworks/plasma-framework!60 (merged)

frameworks/plasma-framework!77 (closed)