[Owncloud] Navigation entry update when enabling app

[Thomas Tanghus]

I was trying to fix oc-1416 "Reload required after enabling an app which 
displays a new tab on the left" and went through most of the 
enabling/installing/removing code in OC_App and OC_Installer, made some 
methods a bit more consistent in what they returned to get the internal string 
app id when installing from apps.owncloud.com which uses numeric ids.

So far so good, but I had forgotten that (most?) internal apps uses 
appid_index as id in the navigation entry. Is there any specific reason for 
that? If not I will change them to use the appid and document that it has to 
be the appid.

Most external apps seems to do that already.

http://bugs.owncloud.org/thebuggenie/owncloud/issues/oc-1416

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Med venlig hilsen / Best Regards

Thomas Tanghus