Show progress when sending a file from desktop

Summary:
Show progress when sending a file from the desktop

BUG: 355044

Test Plan:
Complete transfer:
-Right click on a big file in dolphin
-Select Send to xx via KDE Connect
-Open the Notifications widget and verify progress is shown correctly

Stop transfer:
-Right click on a big file in dolphin
-Select Send to xx via KDE Connect
-Open The Notifications widget and press the stop/kill button
-Observe that the file upload is stopped

Reviewers: #kde_connect, nicolasfella

Reviewed By: #kde_connect, nicolasfella

Subscribers: apol, broulik, nicolasfella, kdeconnect

Tags: #kde_connect

Differential Revision: https://phabricator.kde.org/D16279