Handle switching between different installation of Transmission

When switching between different installation of Transmission,
the base_id of all tasks are completely different and any
old tasks that have existed cannot be reused. Set the base_id
of tasks that no longer exist to -1 so that we can avoid old
data tainting the behaviour after the switch.

Change-Id: Id90052c9c3c3edc7f196c37b12db793094c19333
Allow multiple categories for the same destination folder

Use a list of tuples rather than a dictionary to define
the list of destination directories for categories.

Change-Id: I88756b81b4bd5cdfadbd61143e30d69b44e8f0b6
Support overriding the download root directory

Change-Id: Ie97d67302175e2286fbd54ea0010b6cf28da2990