Better integration of Slip39_Single backup type

[matejcik]

• add tests for Slip39_Single alongside existing tests for Slip39_Basic + 1-of-1 specified in BackupDevice
• handle situation when backup is repeated but stored type is Slip39_Single
  • either we should treat it as "Basic" and allow the user to hand-configure the shares
  • or reject the message
• allow the user to specify a different backup style with BackupDevice, i.e., Advanced or Single when the type is Basic, and so on (let's do this later)
• convert stored type to Slip39_Basic (or Advanced, as appropriate) when doing a basic backup out of a single one

depends on #3825 and #3640