[applets/taskmanager] Add an option to set align direction of items

Sometimes you want to get the tasks in Task Manager aligned to the right, for example, when you use a Global Menu and made it occupy the left spaces.

Well, it is possible to achieve this using the Spacer widget. However, the spacer squeezes the task manager, causing the task manager items to merge even when the panel is not full.

In this merge request, I added an option in the widget config so users can choose the direction task items align on their own.