Added acceptance tests for enabling apps

This tests whether a user can see navigation entries after enabling
apps. This includes the app's group restriction.

This currently expects that a group "group1" exists until we have code
to auto-generate groups.

This commit also provides a utility function
Page.multiselectSetSelection() to make it possible to select entries
inside a multiselect.

@felixboehm @Gomez @DeepDiver1975 

To test, run the suite (replace with your setup values):
```
protractor protractor_conf.js --params.baseUrl="http://127.0.0.1/owncloud/" --params.login.user="admin" --params.login.password="admin" --suite=apps
```
Make sure to have a group "group1" available in your ownCloud.

Note that one test will fail until https://github.com/owncloud/core/pull/10818 is merged.
But I wanted to have these tests reviewed separately. :smile: