applets/taskmanager: Add a new way to style running indicator for themes

Previously, task's background and bar had to be drawn in the same SVG elements, so we couldn't make the bar very short in a proper way.

This allows we to draw the running indicator separately from the background, so we can make it very short (even to a dot).

In addition, this indicator can have styles for grouped tasks.

Theme authors can optionally include a hint-running-indicator-on-screen-edge element in the widgets/tasks SVG to position the indicator on the screen edge side.

This change should not break existing themes.

With the hint-running-indicator-on-screen-edge element:

Here's tasks.svg for testing.