kconfig: menuconfig: simplify global jump key assignment

kconfig: menuconfig: simplify global jump key assignment

Commit 95ac9b3b585d ("menuconfig: Assign jump keys per-page instead
of globally") injected a lot of hacks to the bottom of the textbox
infrastructure.

I reverted many of them without changing the behavior. (almost)
Now, the key markers are inserted when constructing the search result
instead of updating the text buffer on-the-fly.

The buffer passed to the textbox got back to a constant string.
The ugly casts from (const char *) to (char *) went away.

A disadvantage is that the same key numbers might be displayed multiple
times in the dialog if you use a huge window (but I believe it is
unlikely to happen).

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Jesse Taube <Mr.Bossman075@gmail.com>