text: Title: Enabling private sixel color registers should not be necessary
  Body: This is not a big deal, but these private sixel color functions don't actually do anything:
  
  https://github.com/joouha/euporie/blob/be40f57aefad591f8880ea400f64ea7a9ecee924/euporie/core/io.py#L187-L193
  
  Mode 1070 is a private mode, so those sequences should be `\x1b[?1070h` and `\x1b[?1070l`.
  
  That said, it's highly unlikely you need to request private color registers anyway. That's only necessary if you're using an image without defining a color palette, and you want it to use the default palette. But it's not a standard mode, and it won't work consistently even on terminals that do support it, so there's very little justification for using it.