[WIP] Sharing overview page

:construction: 
- added new app bar entry for the sharing overview page (can be moved
  later to the future sidebar)
- use OCS Share API to get the list of files "shared with others" (yay, using our own API!)
- added dummy buttons that will later switch the mode to "shared with
  me"

This is in "prototype" phase.
Note that the mode "shared with me" isn't available yet. @schiesbn will add a new OCS Share API call for that.

@owncloud/designers please try it out and give some input about how the list/table should be displayed.
There is an new navigation item to access that page (will later be rather from the future files sidebar)
![sharingoverview](https://cloud.githubusercontent.com/assets/277525/2843488/25d9506c-d07f-11e3-96d8-29824f2abb04.png)

#### Questions:
1. What do you think about making it possible to switch between "files shared with me" and "files shared with others" ? How to do the switch ? (the prototype has ugly buttons on top)

2. Should "public" and "user/group" shares be grouped into a single row ?

3. What file actions should be available ? I think the share dropdown should be made available here.

4. Currently, clicking on a file will download it and clicking on a folder will jump to the files app. Should clicking on a file also go to the files app ?

5. Should there be a "Download" action ?

6. Should there be a "delete" or rather "unshare" action in "Shared with others" mode ?

7. Should there be a "delete" or rather "unshare myself from this share" in "Shared with me" mode ?

8. Should the column "Share type" be visible at all ?

CC @DeepDiver1975 @karlitschek @craigpg @schiesbn