Ftrack: Delete action revision

[iLLiCiTiT]

Brief description

Ftrack delete action crashes if entity that has children was deleted using ftrack api.

Changes

• find all children of entities to delete and delete them from bottom to the selection
• modified message which will popup when selected ftrack entities does not have avalon equivalent
  • message text: "Didn't found entities in avalon. You can use Ftrack's Delete button fot this selection."

Testing notes:

1. Run delete asset/subset on ftrack entity that has avalon asset and published subsets with asset versions in ftrack
2. Confirm deletion
3. No error should be shown

[mkolar]

Task linked: OP-2310 Ftrack: Delete action revision