Add a QFileDialog::resetSidebarUrls() function

Upon construction, QFileDialog initializes its sidebar URLs from the persistent config file. As a consequence, the sidebar URLs set by a previous instance would be used for the current instance as well. This can currently be worked around by explicitly setting the URLs in the current instance with setSidebarUrls(). A resetSidebarUrls() could be useful (as a special case of setSidebarUrls()) for setting the sidebar URLs to the values that would be default in the absence of a config file (typically / and $HOME on Linux).