Enable file Edit context menu item on Local site window

[Thom Heinz]

Please reference Ticket #3742.
In the context menu on the local site window "Edit" is grayed out when not connected to a remote site but is available when connected. However, editing the file while offline is preferred to editing the file while connected. Proposed workflow:

1. All files stored locally match all online files and folders.
2. FileZilla Client is opened first because if presents a clear directory structure of the local files and folders that need to be edited.
3. Use FileZilla Client and navigate through the local directory structure in the local site window and use context menu to open for "edit" various files while offline.
4. Edit files in "Default editor" as defined in settings.
5. Connect to remote site and upload changed files.

[Tim Kosse]

The whole point of the Edit option is to transfer the files after changes have been made.
For offline editing, use your system's file manager.