Improving code quality for mod_search

This PR started to take care of the blue `No` values in the search module options. I added some other code improvements to it:

- `button` and `imagebutton` parameter now use `0` and `1` as possible values instead of `null` (not set) and `1`.
- `button` and `imagebutton` parameter now consistently default to `0` in XML and code.
- `button_pos` parameter now consistently defaults to `left` in XML and code.
- `set_itemid` parameter now uses the `menuitem` fieldtype. This allows users to select a menu item from the list instead of entering the number manually.
- Improved layout code, moved the `switch` statement into the `if` clause since it's only needed if a button is shown.

This should change nothing for existing sites.