[WK2] Add C API to copy selected files from WebOpenPanelParameters.
https://bugs.webkit.org/show_bug.cgi?id=112339

Reviewed by Andreas Kling.

Replaces the existing WebOpenPanelParameters::selectedFileNames() method
to return a copy of the selected file names and exposes it through the
C API. This is done in order to reduce the direct use of WebKit2 internal
classes. The implementation is very similar to the one in
WebOpenPanelParameters::acceptMIMETypes().

This also updates the GTK port, which is the other user of selectedFileNames.

* Shared/WebOpenPanelParameters.cpp:
(WebKit::WebOpenPanelParameters::selectedFileNames):
(WebKit):
* Shared/WebOpenPanelParameters.h:
(WebOpenPanelParameters):
* UIProcess/API/C/WKOpenPanelParameters.cpp:
(WKOpenPanelParametersCopySelectedFileNames):
* UIProcess/API/C/WKOpenPanelParameters.h:
* UIProcess/API/gtk/WebKitFileChooserRequest.cpp:
(webkit_file_chooser_request_get_selected_files):
* UIProcess/qt/QtWebPageUIClient.cpp:
(WebKit::QtWebPageUIClient::runOpenPanel):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148005 268f45cc-cd09-0410-ab3c-d52691b4dbfc