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/request-plan9.go

@@ -0,0 +1,16 @@
+//go:build plan9
+// +build plan9
+
+package sftp
+
+import (
+	"syscall"
+)
+
+func fakeFileInfoSys() interface{} {
+	return &syscall.Dir{}
+}
+
+func testOsSys(sys interface{}) error {
+	return nil
+}