applets/taskmanager: Add option to switch tasks with scroll on icon hover only

Currently, when scrolling in the Task Manager it will cycle through all tasks, unless the mouse is hovering the icon of an application that has more than one window open, in that case it will focus the application and cycle between tasks of the same group.

This new behavior adds to the current, allowing users to focus another app or cycle between tasks of one group at a time regardless of the amount of windows open, only when the mouse is hovering an application icon in the Task Manager. It matches the behavior of the Dash to Panel extension for GNOME.

Related Bug 461481.

Here's a screenshot of the proposed change:

The label for this checkbox is just a rough idea and should probably be changed to something more descriptive of the actual behavior.

I've been using this for a few days, and it seems to work without any issues.