TaskListPlugin: Use pointers to TaskFiles instead of IDocuments

... in internal list of files we have already open.

Change-Id: I76072c87f374a0881792a04d7ead28e954b94a20
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
TaskList: Do not open the same file several times

This avoids having the issue repeated if you repeatedly opened
the same file.

Change-Id: I998a00b4351ff760a4384dc89a3a7ce94d713500
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>