Newer Trezor firmwares introduced a "seedless" mode.
This is an option during device initialisation. When enabled, the user will never be shown the seed words, and the device even stores this fact; i.e. the user will have "proof" the mnemonic was never shown. Apparently the main use case in mind is redundant multisig setups.
There are now multiple "advanced" options when initialising a Trezor, so I hid some of them behind an "expert" button.
Also did some clean-up.
This is a backport of electrum#5118
with trezor safe 3/5 support from https://github.com/spesmilo/electrum/commit/6172898a035e6ccbe5e8c85a34bc2f14acd106b3
This is a dependency for T3726