Add checkbox or button to empty appCache in Cache API panel

[Jaifroid]

This would mostly be a utility for development, but could also be useful for a user who needs to reset their app to the latest code for troubleshooting.

With the new code as a result of #771, in SW mode (and also in jQuery mode when being run from a secure server) the app will only update its own code if the appVersion number has changed. This complicates development because it is necessary to empty the appCache explicitly in DevTools before reloading the app's page, otherwise new code will not be displayed. A checkbox could be added in the Cache API panel under "Do not cache assets". This checkbox would read "Also empty app cache (refreshes app from server)". The checkbox would only appear if appCache is in use, and would be automatically unchecked if user turns "Cache assets" back on.

For an ordinary user, also, there may be times when the appCache has been filled incorrectly, or partial contents were cached. It would be useful to be able to "reset" the app from the UI.