Assets not included when added when the app is running

[xhluca]

PR Reference: #286

At the moment, the only way to refresh the changes to the assets is to restart the app, and open up a new browser.

[xhluca]

For example, I have a Dash app running with a file called base-styles.css. While the app is running, if I modify base-styles.css to base.css, the following error appear:

builtins.FileNotFoundError
FileNotFoundError: [WinError 2] The system cannot find the file specified: ...