Issues with recent utf8 patch

[Rafał Mużyło <galtgendo o2 pl>]

I decided to give those patches a try and they mostly work.
As it's the first (known to me) release that should work with ncursesw I
tested only that (cause fc slang patches work for me after a simple
correction - they have a problem with double-width chars).
Following issues were found:
1. MC_WITH_NCURSESW is incorrect
If it was a simple copy/paste it would probably work,
however it's not. AC_CHECK_LIB and AC_SEARCH_LIBS differ in a very
important point. AC_SEARCH_LIBS always sets LIBS var with found libs,
AC_CHECK_LIB does that only if no other action is named. As such checks
for ESCDELAY and resizeterm work in MC_WITH_NCURSES but always fail in
MC_WITH_NCURSESW.
2. src/strutilutf8.c has #include <slang/slang.h>
This include is unneeded and breaks compile if slang is not found.
3. internal editor
It's broken (both input and display).
Internal viewer and input widgets seem to work, though.
3. Options/Configuration dialog
Most of the dialogs I managed to check seem fine, this one is, however,
broken.
(May be important LANG=pl_PL.UTF-8)
Nice work, though.