issue_6517: Emoji support

Added issue support for the different output types.

- Sources of the emoji
  - based on the Unicode definition v11.0:
    - https://unicode.org/emoji/charts/full-emoji-list.html
    - http://www.unicode.org/emoji/charts/full-emoji-modifiers.html
  - github definition list:
    - https://api.github.com/emojis

- Input of emoji: :<test>: with the restriction that direct after the opening colon and direct before the closing colon no space is allowed
  - doctokinizer.l, adding detection of emoji and new command `\:`
  - doktokinizer.h, adding "word" type  TK_EMOJI
  - docparser.* handling of new "word" type TK_EMOJI (analogous to HTML Entities), handling of new command `\:`
  - cmdmapper,cpp, cmdmapper.h, adding new command `\:`
  - htmlentity.cpp, adding new definition required for new command `\:`

- Emoji
  - emoji.cpp, emoji.h, class for handling emoji analogous to HTML Entities, including small directions on how to update the code when a new emoji is defined. Not everything is converted to lowercase for comparison and accents are removed.
  - doxygen.cpp possibility to create list of supported emoji

- handling emoji for output types (analogous to HTML Entities), see documentation for different output types
  - docparser.h, *docvisitor.*
  - rtfdocvisitor.* converting output to UTF-16 (based on http://scruss.com/blog/2017/03/12/in-the-unlikely-event-you-need-to-represent-emoji-in-rtf-using-perl/)
  - latexdocvisitor.*, handling arguments for emoji in output (see also latexgen.cpp for meaning of the arguments of doxygenemoji).
  - latexgen.cpp, adding new latex command for doxygen (doxygenemoji) and prevent too many open file (code before documentclass)
  - config.xml, definition of `LATEX_EMOJI_DIRECTORY` with path to images required for LaTeX output

- Documentation:
   - emojisup.doc, user description
   - commands.doc, description of new command `\:`
   - index.doc, reference to emoji chapter
   - xmlcmds.doc, adjust reference to next chapter as a new chapter is added
   - Doxyfile*, adding emoji chapter

Build system
  - CMakeLists.txt adding new files

1 files changed, 9 insertions, 0 deletions

diff --git a/doc/commands.doc b/doc/commands.doc
index 840c3b7..4930524 100644
--- a/doc/commands.doc
+++ b/doc/commands.doc
@@ -214,6 +214,7 @@ documentation:
 \refitem cmdperc \\\%
 \refitem cmdquot \\\"
 \refitem cmdchardot \\\.
+\refitem cmdcolon \:
 \refitem cmddcolon \::
 \refitem cmdpipe \\|
 \refitem cmdndash \\\--
@@ -3362,6 +3363,14 @@ class Receiver
   the start of a line.
 
 <hr>
+\section cmdcolon \:
+
+  \addindex \\:
+  This command writes a single colon (\c \:) to the output. This
+  character sequence has to be escaped in some cases, because it is used
+  to define `emoji` see also \ref emojisup "Emoji support".
+
+<hr>
 \section cmddcolon \\::
 
   \addindex \\::