Support having multiple surfaces per process

Filing the bug as suggested today in #ubuntu-app-devel, not to forget about this use case.

The Online Accounts UI can be invoked by applications in order to ask for the creation of an account or for authorization to use an existing account, and according to the design:
1) the Online Accounts window must be modal to the calling application, and
2) if multiple instances are opened at the same time, they should be able to communicate to each other (so that we can prevent creating the same account in two different apps)

The way I decided to implement this is to have a D-Bus service, com.canonical.OnlineAccountsUi, which is activated upon client requests, creates the needed windows (QQuickView) and sets them as modal on the client. Then the service would exit after a few seconds of inactivity.

For the time being, apart from the modality feature which is not implemented, things work reasonably well; but I'm told that this use-case (having a process create multiple windows) was not considered and keeping it working with the next iteration of unity-mir/unity8 might require some more consideration.