Add start-of-week setting for charts

[schmich]

From Fricka on CWS:

The only thing I would wish to be improved is that I can set the starting day of the week to Monday

From Shin40Spear on CWS:

It would be amazing to add a setting allowing to choose to start the week on Monday instead if Sunday. it would be applied to "Weekly Distribution" and "N Pomodoros in the Last 9 Months" in the History view.

• Setting for first day of week
• Just Sunday/Monday?
• Store integer offset from Sunday (0 = Sunday, 1 = Monday, ...)
• Use setting on history page for stats/charts

[UgurAldanmaz]

Is this issue open to contribution?