Multiple period days marked on one period button click

[dydyamotya]

Hello!

The topic was discussed in discord first, here is the explanation from discord message:

My wife likes the UX in My Calendar application, where you can tap once when the period starts and the app will mark N days as period days, so you don't need to mark each day of your period. Will you accept the PR with such a functionality? I'm thinking of option in setting, that if you select only one day, than the application will mark N days, where N is setup in options/settings.

I've added a new setting: user_defined_period_length
If this parameters > 0, then pressing "Period" button will mark this day and following [average period] days as period days. If average can't be calculated the user provided value is used.

I'm open to discussion. I've started with most simple algorithm I could imagine.
I've tried to figure out all the edge cases.