add: option to import single event share

Hi,

I've been following [this issue](https://github.com/owncloud/apps/issues/203) for several weeks.
And today I got this idea:

If I want to see single events share via caldav, I have to create an event using the shared informations.

So here it is, I added a new 'import' button in the 'showevent' view so that you can create your own event using the shared informations.

It is not the best we can do, but I hope it's a start.