xhtmlxtras plugin new button using html quote tag

Hello,

I add a new button to the xhtmlxtras plugin. This button uses xhtml quote tag <q></q> and  it is based on the same principles than cite button.
It's the first time i patch an application and i use github so if you find mistake, tell me and i'll try to correct it.

Thanks for your help.

Best regards,

Florent Jouatte.