Separate task import from task serialization

Right now the task definition is serialized as XML file on disk and is considered immutable.
There are several issues associated with this solution:

• inefficiency (parsing of xml when loading serialized task),
• would not be a part of serialized package if the serialization engine changes (e.g. to sqlite db).
• does not allow changing the task definition (for example by changing the source drive)
• to make some parts of the task definition mutable, they are copied to other location (some member variables) and are also serialized.
• operation on xml's would be better described as import/export than serialization.

With this story, we'd like to change the serialization of xml to import (possibly also export) operation along with:

• moving the import functionality to a separate object (task importer instead of task definition load).