Don't cancel and create new timers naively.
Only cancel timer (and set to nil) when there is no live buffer
with eldoc-box-hover-mode enabled;
and only create new timer when none exists.
Add cleanup function and a idle timer
Why a timer? ElDoc is mainly use in minibuffer,
where the text is constantly being flushed by other commands
so ElDoc doesn't try very hard to cleanup
Originally I have (when str (display)), but that leaves childdrame
unchanged.
To work around that I added (eldoc-box-quit-frame).
Now childframes are cleared up nicely but it lagging.
(when (stringp str) (display)) works. (although I don't know why (when
str) doesn't, even though eldoc-messaeg passes nil to message function)
