[Widgets] wire the file pickers

Introduce a new version of chooseFiles in QWebEnginePage.
The existing API in WebKit1 seemed a bit dusty in any case (multiple
only supported via extensions).

Changes are:
  * oldFile becomes oldFiles, so that we could at a later stage expose
    the already selected files in the "multiple" case.
  * a type is introduced, for now limited to multiple selection, but
    over time, we might consider additions such as directory upload.

Change-Id: I14cfea64ce95e892a0a1877c8cb914c5a421409f
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>