UX: reintroduce our custom screensaver

[spalmer25]

Context

Since commit a21e7b7, the baking application is no longer able to sign baking operations quickly enough.
The signing time for this commit is identical to the previous one (1-2s) but frequently the signing time could increase by 3-6s.

In this commit, our custom io_event function, which overrides the one in Ledger's lib_standard_app, has been removed.
The difference between these two versions of io_event is that TICKER events are not handled in our version.
These events have been reactivated in this commit so that Ledger's screensaver can be used.

But it seems that handling TICKER events slows down the application.

Goal

Stop to handle the TICKER event again and reintroduce our customised screensaver.