Don't steal focus after detecting file changes and deletions

[darahak]

Fixes #132.

Now the dialogs are displayed only when the app gets the focus back.

Also because it is an async matter now, when a dialog is about to be displayed, it checks if the file still exists or not.
For example, if the file is modified by a third-party, and then deleted, only the "Delete" dialog will show up.

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• You worked on the develop branch (or any other branch which was forked from develop),
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• You tested your code and it works.

Thank you!

[brrd]

This is perfect now! 👍