wallpapers/image: use url argument type in `addUsersWallpaper`

The hash sign # will be converted to % when using string argument type.

CCBUG: 454692

wallpapers/image/plugin/imagebackend.cpp

@@ -518,9 +518,9 @@ void ImageBackend::slotWallpaperBrowseCompleted()
     }
 }
 
-QString ImageBackend::addUsersWallpaper(const QString &file)
+QString ImageBackend::addUsersWallpaper(const QUrl &url)
 {
-    auto results = static_cast<ImageProxyModel *>(wallpaperModel())->addBackground(file);
+    auto results = static_cast<ImageProxyModel *>(wallpaperModel())->addBackground(url.toLocalFile());
 
     if (!m_usedInConfig) {
         m_model->commitAddition();

wallpapers/image/plugin/imagebackend.h

@@ -84,7 +84,7 @@ public:
 
     Q_INVOKABLE void showFileDialog();
 
-    Q_INVOKABLE QString addUsersWallpaper(const QString &file);
+    Q_INVOKABLE QString addUsersWallpaper(const QUrl &url);
 
     Q_INVOKABLE void useSingleImageDefaults();