Fix compilation with UTF-8 wxString
Add missing include
Add a command to cycle between subtitle providers
Improve some help text for commands

Consistently use the imperative mood and expand some overly terse help
messages.
Remove entirely superfluous doxygen comments in commands
Remove the help/files command

The local docs it tries to open don't actually exists and if they're
ever added, the help contents command opens them anyway so I'm not sure
what the intended purpose of the command ever was.
Update the manual URL now that the 3.1 manual partially exists