emojis and other multi-character glyphs

[Evgeny Zajcev <lg.zevlg@gmail.com>]

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There is some inconsistency in naming and behaviour in Emacs master.
We have `forward-char', `backward-char', `delete-char',
`backward-delete-char' commands.  All of them use "char" in their names,
however, `forward-char' and `backward-char' treats "char" differently than
`delete-char' and `backward-delete-char'.

Let me explain.  Emacs has support for composed characters to display
multiple characters composed into a single glyph.  Almost the same is done
for multi-character emojis such as 🇷🇺 or 👨‍👩‍👧‍👦 - multiple unicode
chars are composed into single glyph representing some emoji.  Now, if you
put point under composed character or emoji and run `forward-char' or
`backward-char' it moves point to the whole glyph, however, if you run
`delete-char' (when point is under composed char) or
`backward-delete-char'(when point just after the glyph) it will delete only
single character from multiple character representation, so pressing `C-d'
under 🇷🇺 will magically turn Russian flag into 🇺.  This is very
misleading behaviour especially when invisible characters are used in the
emojis

Maybe introduce "glyph" term meaning graphical representation of chars
sequence, displayed in the buffer and operated as a whole thing?

For example these things creates a glyph in a buffer:
1) (compose-chars ?a ?b)
2) (concat "\x1F1F7" "\x1F1FA")
3) (propertize "aaaa" 'display "B")

In this case, we can rename `forward-char' to `forward-glyph' keeping
naming and behaviour in consistency.

And also it will be possible to write something like `string-glyph-length'
to return 1 for "👨‍👩‍👧‍👦" instead of 7 as `length' returns now.

What do you think?

Thanks

-- 
lg

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