applets/taskmanager: port TaskList to ListView

Main changes:

• Task geometry properties are moved to TaskList, and LayoutManager is greatly simplified and becomes a library.
• When maxStripes > 1, there is no scrollbar and the old behavior is kept.

A few visual differences:

• Task animation looks less elegant than before
• The minimum width of a task is fixed, and no longer affected by the number of opened tasks.

Before
After