When file is open user should be prompted if he wants to execute certain actions

[pixtron]

Describe the change

If one of the actions delete, restore version and revert event is triggered, while a file is open the user should be prompted if he want's to execute the action.

Also it should be checked:

1. if there is any unsaved content
2. if the file is locked and the lock has to be released

Current situation

The user is alerted that the operations can't be executed when a file is open.

Should

File handlers should return a close handle, so a file can be closed from outside the app that it opened. The corresponding app should handle the necessary unlocking, dirty checking etc.

Also openFile should store the currently opened file, but only for fileHandlers which actually open a file (eg. not preview, not download)

Additional context

This is a follow up issue for #206.