Move po generation from Make to CMake.

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.

cmake/ConvertPo.cmake

@@ -0,0 +1,21 @@
+string(TOUPPER ${INPUT_ENC} upperInputEnc)
+string(TOLOWER ${INPUT_ENC} lowerInputEnc)
+get_filename_component(inputName ${INPUT_FILE} NAME)
+execute_process(
+  COMMAND ${ICONV_PRG} -f ${INPUT_ENC} -t ${OUTPUT_ENC} ${INPUT_FILE}
+  OUTPUT_VARIABLE trans
+  ERROR_VARIABLE err
+  RESULT_VARIABLE res)
+if(NOT res EQUAL 0)
+  message(FATAL_ERROR "iconv failed to run correctly: ${err}")
+endif()
+
+string(REPLACE "charset=${lowerInputEnc}" "charset=${OUTPUT_CHARSET}"
+  trans "${trans}")
+string(REPLACE "charset=${upperInputEnc}" "charset=${OUTPUT_CHARSET}"
+  trans "${trans}")
+string(REPLACE "# Original translations"
+  "# Generated from ${inputName}, DO NOT EDIT"
+  trans "${trans}")
+
+file(WRITE ${OUTPUT_FILE} "${trans}")