Implement morekeys

Associate morekeys with keys.
Implement morekeys view
Implement morekeys popup on longpress

On key longpress, show a popup window of morekeys, along with a close
button for the popup window.
Provide morekeys on enter keys

For the enter keys in Three Corner and qwerty keyboards, provide
morekeys actions.
Provide morekeys hint on key-top

For keys which has morekeys, provide key-top hints (i.e. "...") and key
preview hints.
Implement editor actions

Implement editor actions such as send, next, done, etc.
Remove useless symbol mode ID