File Open dialog issue

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I'm having issues combining File Open dialogs and timers in the druid-shell hello example.

Currently File Open dialogs block while holding a mutable borrow on the `WinHandler` until the dialog comes back with a result path. If a timer event triggers while the dialog is open, it panics with "already borrowed".

#95 mentions a possible solution. Are there other solutions to consider?

As a workaround for now I don't block when opening the dialog, immediately returning an Err(Error:Null) in `get_file_dialog_path` and I have a closure that runs with the actual result but I'm not sure where to send it. If a solution is decided upon I can take a shot at a PR.

Start with the druid-shell hello example and inspect get_file_dialog_path, WinHandler borrowing, and the timer path described in the issue. Read issue #95 and compare possible approaches for handling a timer event while a file dialog is open. Done means an agreed solution is clear enough to implement and the dialog result has a defined destination.

Written by the indexing model from the issue text.