sort order of tasks

[julianpoemp]

• add sortorder attribute to IDB
• unsorted is -1
• the sortorder represent the index position in the list of tasks
• to avoid calculation overhead, the sortorder is saved whenever a task or directory is added/inserted
• change sort order via context menu on the proceedings table
• change sort order via arrows in the queue modal

[julianpoemp]

related to #35 #24