Fix Bug 412341 - add option to specify dependencies manually

Adds a dedicated dialog to task editor and dependency editor.
Makes it possible to add predeccessors to the selected task.

Typical use case:

1. Add a task
2. Open the dependency dialog
3. Add the dependency/dependencies (or just accept to connect to previous task)

By default a dependency on the previous task is presented.
The task dropdown list is sorted with prevoious tasks first.

BUG: 412341
FIXED-IN: 3.1.89