Use QUrl::fromLocalFile to convert filenames

Summary:
Allows importing/exporting local files in KisImportExportManager.

Importing/exporting .png, .jpg and so on were broken on my machine following the port to QUrl, because the to/from string functions became less forgiving. For example QUrl("/home/miabrahams/test.jpg") is a no-go, it must be of the form "file:///home/miabrahams/test.jpg". Similarly QFile(QUrl.toString()) will fail.

Reviewers: rempt

Reviewed By: rempt

Differential Revision: https://phabricator.kde.org/D356