Reworked the folder representation, and fixed/tested the application
features.
Improved: reworked the folder representation; folders are now combined
from all CSPs if they have the same name, and their updates are event
based.
Improved: refreshing the tree is faster, and loading it doesn't hang the
interface.
Improved: file upload chooses the best fitting CSP, and creates a folder
if necessary.
Improved: creating/deleting a folder is done on all CSPs.
Unchanged: deleting/copying/moving a file is done on related CSP only.
As cross-CSP operations isn't officially supported, so in the future
I'll create a service on a private server to support it.