Feature: When file being edited has unsaved changes, ask for confirmation before closing the edit dialog

[awa-xima]

ElFinder includes many editors for editing files directly within the file explorer. When a file has unsaved changes and the user closes the edit dialog, ElFinder currently first closes the dialog, then asks the user whether they want to save their changes:

confirmAfterClose.webm

We received feedback by our users that this is unintuitive. Instead, ElFinder should keep the dialog open and ask the user whether they want to cancel the operation (keeps the edit dialog open), discard their changes, or save their changes. This is also the behavior of many other text editors. To illustrate, ElFinder should behave like this:

confirmBeforeClose.webm

I'll open a PR for this.