FR: Put power search selection on main search page

Power search can be enabled in the settings. However, you have to decided which search you want there. It would be useful to have the power search directly on the main search page.

Here some ideas how to do it:
- Either use the arrows which now select between names and content to go to a third option "power search"
- Offer the check button for power search on the main search page
- Make a second search field for power search