Add edit actions to table context menus

[svetter]

Present if breadcrumbs connection to item is forward only, enabled if item IDs are all set along the path

• Refactor: Create new widget class for item table tab contents #218
• During tab content initialization:
  • Find out which other types are reachable by forward-only breadcrumbs
  • For each resulting type, add an action to table context menu
    • Set action icon per type
• On table context menu open
  • Find out whether there is an unbroken chain of set foreign IDs that determine a valid item ID for each type with an edit action
  • Set each action enabled iff so
• On context menu click
  • Open item dialog of the appropriate type and with the appropriate init data
• Test