SSH connection status indicators

- Add Connected bool field to vfs.Entry and RemoteMount
- Track connection status in sshState.connectedHosts
- Show status icon (connected/disconnected) in pane header when browsing remote host
- Async SSH connection test with cancel support for Add Host dialog
- Colored labels and styled help text in SSH dialogs
- Confirmation dialog when deleting manually-added SSH hosts

vendor/github.com/pkg/sftp/attrs_stubs.go

@@ -0,0 +1,12 @@
+//go:build plan9 || windows || android
+// +build plan9 windows android
+
+package sftp
+
+import (
+	"os"
+)
+
+func fileStatFromInfoOs(fi os.FileInfo, flags *uint32, fileStat *FileStat) {
+	// todo
+}