[TL 21] Select and remove multiple groups

[flomich]

User Story

As a User,
I want to select and remove multiple groups
so that I can quickly filter my list of upcoming tasks.

Acceptance Criteria

• Users can open the side window and click on a group.
• The side window closes and the task list is shown again.
• At the top, there is a small icon with the group name and background color which has a small "x" next to it.
• The icon indicates that only tasks from that group are currently shown.
• The user can click on the small "x" in order to remove the group filter and display all tasks again.
• Users can also open the side window again to select another group.
• The task view shows all tasks of all selected groups in chronological order, each of them can be removed by clicking the "x" icon.