Mixed indentation is unclear

The "Mixed" tab policy is unclear and undocumented. I have no idea what it means, and it looks like it just replaces tabs with spaces.

I found a similar QTCREATORBUG-11675 that was reported long ago.

It could be really useful to have a real "Mixed" policy, that uses tabs/spaces according to the surrounding lines.

I know it's bad, but we have files with both spaces and tabs in the same file, and when I edit such a file, unless I want to reformat an entire section, I have to play with the settings back and forth, or to just disable "Clean indentation".

Also relates to QTCREATORBUG-31545.