The Calendar, Memos and Tasks views use to do D-Bus calls to
the backends on the main (UI) thread, which could result in UI
freezes, until the operation was done on the backend (and server)
side. This commit fixes that by invoking the operations in
a dedicated thread. It has few additional advantages too:
- operations can be cancelled
- proper error reporting to a user
- less code duplication between the views for common operations

There had been fixed some performance issues when selecting/unselecting
sources in the source selector as well.