Return a list of symbols representing the modifier keys in event EVENT.
The elements of the list may include `meta', `control',
`shift', `hyper', `super', `alt', `click', `double', `triple', `drag',
EVENT may be an event or an event type. If EVENT is a symbol
that has never been used in an event that has been read as input
in the current Emacs session, then this function may fail to include the `click' modifier.