(quail-translation-keymap): KP_Enter key

(quail-translation-keymap): KP_Enter key
emulates `C-SPC'.  `mouse-2' bound to
`quail-mouse-choose-completion'.
(quail-completion-list-translations): Set text property
`mouse-face' of character `translations' to `highlight'.  Changed
`newline' to `insert "\n"' to prevent text property inheritance.
(quail-mouse-choose-completion): New function bound to `mouse-2'
selects highlighted characters from *Quail Completions* buffer.
(quail-choose-completion-string): New function called by
`quail-mouse-choose-completion'.