[GSOC] CMIS updates

Hi all,

This is a progress update of my GSOC project concerning CMIS support.
The last weeks I've managed to:

- Fix some bugs in the properties dialog UI.
- Enable CMIS properties updating.
- Add Google Drive support in LO code.

The code can be found in the branch feature/cmis. Feel free to test it,
however you will also have to build libCMIS.

I currently have some problems as following, it would be great if anybody
has a hint to fix them faster :-)

- Refine the UI, including conversion from .src to .ui:
   + The current UI uses a Windows to support mouse scroll, I'm still not
sure how to convert a Window to .ui?
   + How can we convert a HeaderBar to .ui file?

- The file picker has some problems with object ID of a file which prevents
a correct navigation. I will have to change to using object path ( which
doesn't support in Google Drive) or fix those problems.
- A property of CMIS file can have multiple values, need to add support for
that.
- in libCMIS, move to use libboost for JSON parser instead of libjson to
reduce dependencies.

Bests,
CC Ngo.