Added choosing first day of the week (#19)

Added a dialog for choosing first day of the week to any day.
By default, first day of the week is:
- If a user in the old version of the app had set an option that Sunday should be first, Sunday will be set as a first day. Code for that is in MainActivity and can be removed in the next version after releasing this one.
- If there wasn't anything set earlier, it will use the default first day of the week for current locale (same as in Calendar).
Modified the sorting by day logic to consider different first days of the week. Also, simplified that logic a bit.
Modified the alarm edit dialog to sort days properly.

FossifyOrg / Clock