klipper: ignore file previews from network mounts
Reason for the change
A remote desktop application like RustDesk that relies on fuse to support seamless clipboard might leave a local file url in the clipboard but the file is actually a remote file. If there are many urls like that and those remote files are big enough, plasmashell will hang in the end.
QUrl::isLocalFile doesn't check if a file is on a network mount. Instead, use KFileItem::isSlow which has a more comprehensive logic to detect remote files.
Test plan
- Use RustDesk to connect to a remote Windows desktop
- Enable “Enable file copy and paste” which should be enabled by default
- Copy a lot of big files in the remote desktop
- plasmashell should not hang after a while