Mercury PIN keyboard exit flow

[obrusvit]

Edit: this behavior will be used only in initial PIN setup, not generally. So this PR is postponed for now.

This PR replaces request_pin with flow_request_pin. The added benefit is that hitting 'x' button does not dismiss the keyboard immediately but rather opens a VerticalMenu with "Cancel" option. Avoiding the abrupt closing of the PIN keyboard should improve the UX somewhat.

In order to do that, this PR also introduces FlowMsg::Text(ShortString) variant which enables flows to send out strings to micropython.

Takes 2 KiB of flash.

TODO:

• [ ] modify input_flows for cancel PIN