Add clear cache option from the Apple settings

[giomfo]

To deal with the migration from v1.2.3 to v2.0.x, we consider useful to trigger a clear cache when the application launch again after a crash.

This systematic clear cache mecanism should be optional by a configuration flag (because clear cache may have cost (data consumption) for the end users)

[phloux]

We discussed about another mechanism :
We should add a Settings bundle to the app, with a switch to let the user do a manual clear cache on application startup.

[giomfo]

@phlniji I don't see the option in the system settings, where it is supposed to appear?

[phloux]

It should be on System Settings, but might be missing on version 2.0.1 (1). We should rebuild the 2.0.1 for a second build to fix it, or for the 2.0.2 version.