glib: keep same visual appearance between displayed and copied text

When copying text from displayed document to the clipboard,
we want a normalization that preserves 'canonical equivalence'
i.e. that the text after normalization is not visually
different than the original text. Our previous normalization
was just preserving unicode 'compatibility'.

Relevant documentation:
* https://www.win.tue.nl/~aeb/linux/uc/nfc_vs_nfd.html
* https://en.wikipedia.org/wiki/Unicode_equivalence
* https://developer.gnome.org/glib/stable/glib-Unicode-Manipulation.html#g-utf8-normalize

Issue #724
7 jobs for BUG_fix_glib_normalization in 6 minutes (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #7296281
build

00:06:00

passed #7296286
build_android

00:00:37

passed #7296282
build_clang_libcpp

00:04:40

passed #7296285
build_clazy

00:05:34

passed #7296284
build_mingw64_fedora

00:03:03

passed #7296283
build_ubuntu_16_04

00:03:37

passed #7296280
clang_format

00:01:24