Make search in "Lnaguage Shared" folder work

When using the search viewlet, or the search form, the path parameter is a string, not a dictionary having a "query" key with a list of paths as a value.

We now check first to see if query['path'] is a string to make the search form work on the /shared folder also.

The first commit (Flake8) is also in https://github.com/plone/plone.app.multilingual/pull/107.