Reduce the number of activity types

[nickvergessen]

https://github.com/libasys/calendarplus/blob/master/lib/activity.php#L57-L67

11 types is way too much, I think most users don't change them individually, so my suggestion would be:

        self::TYPE_SHARED_EVENT => $l->t('An <strong>event or todo</strong> has been <strong>modified</strong> on a shared Calendar'),
        self::TYPE_SHARE => $l->t('An <strong>event, todo or calendar</strong> has been <strong>shared/unshared</strong>'),
        self::TYPE_EVENT => $l->t('An <strong>event or todo</strong> has been <strong>deleted</strong>'),
        self::TYPE_CALENDAR => $l->t('A <strong>calendar</strong> has been <strong>modified</strong>'),

whereby modified means: created, edited or deleted.

@jancborchardt

Related to owncloud/activity#356

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[farson2003]

Please correct the spelling: In english it should be "An event..." and not "A event".