5.0 prerelease

[GoldenSoju]

Solves #298 (Change of actual RecurrenceRule Class).

Breaking Change:

• Replace old Recurrence Rule implementation with rrule package.

Other Changes:

• Use of Flutter Lints
• Add event status
• Several dependency updates
• Sample App updates/improvements
• Other fixes/improvements

Needed before final release:

• Readme/Changelog update

Should we do a prerelease (for one week maybe)?

[thomassth]

I was thinking 4.3 but that works too

I think we can do 2 weeks prerelease just to be safe

Also, I was thinking about adding some compatibility to the old recurrence rule, to make the switchover easier