Allow notification to be dragged and dropped (#473)

* Set x-kde-urls for notifications

The x-kde-urls field in the hints map allows us to provide a direct URL
to the saved screenshot. The notification server may then use this hint
to provide extra functionality, such as drag and drop support and
thumbnails.

For more information see https://community.kde.org/Plasma/Notifications#File_Path

Fixes #256

Note that other notification servers may ignore this hint.

* Avoid branching twice for ok

src/utils/screenshotsaver.cpp

@@ -41,15 +41,17 @@ bool ScreenshotSaver::saveToFilesystem(const QPixmap &capture,
     completePath += QLatin1String(".png");
     bool ok = capture.save(completePath);
     QString saveMessage;
+    QString notificationPath = completePath;
 
     if (ok) {
         ConfigHandler().setSavePath(path);
         saveMessage = QObject::tr("Capture saved as ") + completePath;
     } else {
         saveMessage = QObject::tr("Error trying to save as ") + completePath;
+        notificationPath = "";
     }
 
-    SystemNotification().sendMessage(saveMessage);
+    SystemNotification().sendMessage(saveMessage, notificationPath);
     return ok;
 }
 
@@ -80,7 +82,7 @@ bool ScreenshotSaver::saveToFilesystemGUI(const QPixmap &capture) {
             QString pathNoFile = savePath.left(savePath.lastIndexOf(QLatin1String("/")));
             ConfigHandler().setSavePath(pathNoFile);
             QString msg = QObject::tr("Capture saved as ") + savePath;
-            SystemNotification().sendMessage(msg);
+            SystemNotification().sendMessage(msg, savePath);
         } else {
             QString msg = QObject::tr("Error trying to save as ") + savePath;
             QMessageBox saveErrBox(