2 main tabs: Add tab with results of individual tasks

Previously part of the bottom right widget. "Quick overview which turned out especially badly"

On Main window 2 tabs. First is the main tab, and one for the tasks statistics. In tab for tasks:

• The tab should contain a drop bar to select a task
• The selected task shall be displayed with a histogram showing how many people got how many points (same as on main window the overall point histogram)
• Additionally: A vertical line showing the average points (or just a text field if too much work for now=)