WIP: Modification for the working of social-help activity.

This PR introduces the changes required in sugar for the functioning of the social-help activity. The changes are self explanatory and I'm adding a few comments (GitHub ones) in the diiff to explain some of the things. 

This PR is a submission to one of the GSoC 2014 sugarlabs projects.

PS: I still have to run pep8 and pyflakes on all files.
Actually, I wrote about this problem in the mailing list (https://www.mail-archive.com/sugar-devel%40lists.sugarlabs.org/msg37842.html). Simply put, here's what we're doing now:
- Don't display the launch button if the activity doesn't have a category.
- If shortcut is pressed in such an activity, do nothing - just as sugar-help does.

I don't have an activity instance do show the notification and that's my problem. Unless there's some method around this, I'm at a loss. Though maybe making a wiki page on the server about the absence of the category might be a good idea instead of the notifications.