Check for missing files before export

[bmatherly]

Fix #990: When deleting a source file, nothing alterts me when
exporting that a source file is missing

[bmatherly]

I also considered moving the entire check up into the main application and adding a menu option so that the user can run the check whenever they want. But then I thought that we already do the check every time the file is opened. So that should be good enough. But, unfortunately, that means that the only way a user can trigger a repair is to close and reopen a project.

What are your thoughts about moving it into the main application?

[bmatherly]

@ddennedy Another thought occurred to me. Would it work to scan the auto-save file before exporting? Upon clicking the export button, Shotcut could create a new auto-save file and scan that. But I do not know if an auto-save file always exists. Thoughts?

[ddennedy]

The autosave file approach would not be sensitive to the Export > From field as you have it now.

[bmatherly]

The autosave file approach would not be sensitive to the Export > From field as you have it now.

That is true. It would be checking the entire project as it does when a project is opened. I guess there are pros and cons to that.