[snippet] Everything but the @prefix keyword should be optional in the @prefix directive

When a user wants to make use of prefix autocompletion, he only has to
press a single key (delete) in order to clear out the remaining line.
[cosmetics] Apply final corrections pointed out by @Infininight

* The standard shortcut for completions is ⌥⎋
* The standard shortcut for documentation is ⌃H
* The 'About' command should be titled 'Help'
* The scope for 'Symbol list: Shown' is not specific enough
* Grammar should not have a scope selector, this stops the key
equivalent from working.
* keyword.directive.* should be keyword.other.directive.*
[autocomplete] Add icon for Prefix
[autocomplete] Re-enable autocompletion for XSD Datatypes