add win-app-bar-separator directive

 *  ...as win-app-bar-command with type="'separator'" does not work
    due to it using <BUTTON /> instead of <HR /> in the template as
    required for separators.

Signed-off-by: Dylan Borg <borgdylan@hotmail.com>
Merge pull request #47 from borgdylan/borgdylan/handles

add win-app-bar-separator directive