Summary:
Allow in progress file transfers to be canceled

BUG: 349956

{F6373048}

{F6373050}

{F6373051}

Test Plan:
Send a large file from desktop to android
Press cancel in the progress notification

Result: the file transfer is cancelled and the cancelled file is deleted from storage

Reviewers: #kde_connect, nicolasfella, albertvaka

Reviewed By: #kde_connect, albertvaka

Subscribers: albertvaka, nicolasfella, kdeconnect

Tags: #kde_connect

Differential Revision: https://phabricator.kde.org/D16491