Fixes #902: localization build: `install`, `uninstall`, `prefixcheck
targets.  All the language-related bits will now build under CMake.
Changes include:

  * Moving all non-generated sources into the NEOVIM_SOURCES variable
    to aid in generating the .pot file.
  * Moving a couple generated sources from NEOVIM_SOURCES and into
    NEOVIM_GENERATED_SOURCES.
  * Added NEOVIM_HEADERS to the executable and the library for folks who
    are using something other than Ninja or makefiles (that way the
    headers will show up in the IDE files).
  * Now uses gettext's `--update` switch to update the .po files, rather
    than doing a fragile `mv` dance that could leave you with a broken
    working tree if you press CTRL-C at the right time.
  * Creates `update-po-${LANG}` targets for updating individual
    languages, just like the original Makefile.
  * Also adds the `update-po` target for updating all the languages.
  * Ported the `check-${LANG}` style targets.  They're
    `check-po-${LANG}` under CMake.
  * Handles all the one-off instances that were in the original
    Makefile.  Fixed up ko.UTF-8.po to include the "Original
    translation" line like other .po files to make the generation of the
    "Generate from ..." comments consistent.  Updated ko.po with the new
    text.

This reverts commit 53fba180c32724809d8ab35238eea68440b7ac97, reversing
changes made to a3b3db8256c259d10957d45666cf4179e328f3f4.

Fixes #902: localization build: `install`, `uninstall`, `prefixcheck
targets.  All the language-related bits will now build under CMake.
Changes include:

  * Moving all non-generated sources into the NEOVIM_SOURCES variable
    to aid in generating the .pot file.
  * Moving a couple generated sources from NEOVIM_SOURCES and into
    NEOVIM_GENERATED_SOURCES.
  * Added NEOVIM_HEADERS to the executable and the library for folks who
    are using something other than Ninja or makefiles (that way the
    headers will show up in the IDE files).
  * Now uses gettext's `--update` switch to update the .po files, rather
    than doing a fragile `mv` dance that could leave you with a broken
    working tree if you press CTRL-C at the right time.
  * Creates `update-po-${LANG}` targets for updating individual
    languages, just like the original Makefile.
  * Also adds the `update-po` target for updating all the languages.
  * Ported the `check-${LANG}` style targets.  They're
    `check-po-${LANG}` under CMake.
  * Handles all the one-off instances that were in the original
    Makefile.  Fixed up ko.UTF-8.po to include the "Original
    translation" line like other .po files to make the generation of the
    "Generate from ..." comments consistent.  Updated ko.po with the new
    text.

Problem:  On OSX, sed commands processing files converted to encodings
          other that UTF-8 fail with "RE error: illegal byte sequence".
Solution: Make sed execute with C locale throgh environment variables
          (LANG=C, LC_ALL=C, LC_CTYPE=C).

Problem  : Previous build assumed all *.c files were in parent dir. It
           only included globals.h, too.
Solution : Include all *.c and *.h files under parent dir (including
           subdirs).

Problem:  Makefile references some files that don't exist anymore.
Solution: Remove those references.

- Fix executable path.
- Make po file title similar as others.

Move files from src/ to src/nvim/.
- src/nvim/ becomes the new root dir for nvim executable sources.
- src/libnvim/ is planned to become root dir of the neovim library.