Add accesskeys to the datepicker container buttons

[jaller94]

As it had been mentioned in #157, I would prefer keyboard shortcuts which do not require combinations. There are also accessibility issues known when using accesskey described on MDN:
https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/accesskey

Still, I have the opinion that this would be a great quick fix to make some sort of keyboard shortcuts available until there is a JavaScript handling available.

• P for the previous period
• N for the next period
• D for the day view
• W for the week view
• M for the month view
• T for the current day (today)

[tcitworld]

I'm fine with having this for the time being, but it's kind of useless unless documented.

[jaller94]

I noticed that Nextcloud Mail has a page for keyboard shortcuts. It is hidden in the settings.
https://github.com/nextcloud/mail/blob/master/js/templates/keyboard-shortcuts.html

And from the main app I found a file that handles most of the single key shortcuts:
https://github.com/nextcloud/server/blob/master/apps/files/js/keyboardshortcuts.js

[georgehrke]

Closing because of the restrictions with accesskey.
This needs to be reimplemented in the Vue branch and we should use that opportunity to use some proper Vue Plugin for that.