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/README.md

@@ -0,0 +1,44 @@
+sftp
+----
+
+The `sftp` package provides support for file system operations on remote ssh
+servers using the SFTP subsystem. It also implements an SFTP server for serving
+files from the filesystem.
+
+![CI Status](https://github.com/pkg/sftp/workflows/CI/badge.svg?branch=master&event=push) [![Go Reference](https://pkg.go.dev/badge/github.com/pkg/sftp.svg)](https://pkg.go.dev/github.com/pkg/sftp)
+
+usage and examples
+------------------
+
+See [https://pkg.go.dev/github.com/pkg/sftp](https://pkg.go.dev/github.com/pkg/sftp) for
+examples and usage.
+
+The basic operation of the package mirrors the facilities of the
+[os](http://golang.org/pkg/os) package.
+
+The Walker interface for directory traversal is heavily inspired by Keith
+Rarick's [fs](https://pkg.go.dev/github.com/kr/fs) package.
+
+roadmap
+-------
+
+* There is way too much duplication in the Client methods. If there was an
+  unmarshal(interface{}) method this would reduce a heap of the duplication.
+
+contributing
+------------
+
+We welcome pull requests, bug fixes and issue reports.
+
+Before proposing a large change, first please discuss your change by raising an
+issue.
+
+For API/code bugs, please include a small, self contained code example to
+reproduce the issue. For pull requests, remember test coverage.
+
+We try to handle issues and pull requests with a 0 open philosophy. That means
+we will try to address the submission as soon as possible and will work toward
+a resolution. If progress can no longer be made (eg. unreproducible bug) or
+stops (eg. unresponsive submitter), we will close the bug.
+
+Thanks.