…y_external_outputs functions

…using NBGL.

If a transaction has only a single non-change output, then we can fit the
"Amount", "To" and "Fees" fields in a single screen.

By caching the first few external outputs during output preprocessing, we can greatly
simplify the handling of the swap-related code, and the simplified UX for a single
external output.

This also allows to move all the swap-related checks in a single function.
Finally, by moving all the checks not related to user approval out of show_alerts
and display_output, we can omit calling these functions at all during swaps.

… transaction flow on flex/stax

Various simplifications in the code to generate the ragger instructions.

…tead of waiting for the timeout

… remaining flows to use 'accounts' instead of 'wallet policy'

Minor adjustments needed in some tests as the page splitting changed.