[Direct Transfer] Display a remote folder icon for each transfer

Display the remote folder for each transfer using a folder icon on the left of the progress bar, instead of having a "header" saying "Senting files to xxx".

This will enhance the overall experience:

1. When a DT is ongoing, and one uses the OS context menu entry to send more files into a different remote folder, then the header will be udated with the new remote folder. It is confusing as one might think that the remote folder will be changed for current transfers too.
2. Indirectly, this will fix a performance issue when sending 1000+ files. For each and every file: when a chunk is uploaded, if the upload state changed or if it is completed, then a signal is sent to the UI to update the remote folder in the header. This is incredibly unperformant but it was done on purpose to keep the header up-to-date. And this implies a lot of communication between QML > C > Python "just" for that information.

So, storing the remote folder title and ref for each transfer will be more reliable and fix performance issues.