Accept "application/ics" in ImportActivity intent-filter

While text/calendar is the correct mime type for .ics files, there are
some non standard-compliant implementations out there, such as Gmail's
invitations that use application/ics as the mime type and thus don't
work with Etar.

Way to reproduce:
1. Get someone that uses Gmail to send you an invitation to an event.
2. Try opening the event directly from a mail app that just uses the sent
    mime type as sent in the email.

Before this patch:
1. Etar won't be opened.

After this patch:
1. Etar is opened.