Add suport for ICS calendar files

[eltos]

Add support for ICS calendar files, e.g. to copy an appointment from Thunderbird calendar and paste it into a file (or vice verca)

Specification and constraints

• File endings: .ics
• Mime: text/calendar
• Also plain text if it starts with BEGIN:VCALENDAR

See https://de.wikipedia.org/wiki/ICalendar