D19052: Fix captions with non-BMP characters

cfeck created this revision.
cfeck added a reviewer: KWin.
Herald added a project: KWin.
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cfeck requested review of this revision.

REVISION SUMMARY
  KWin replaces any non-printable character with a space. This check does not handle surrogate pairs correctly. Additionally, translators sometimes insert non-printable soft-hyphens into titles, which also cause KWin to display a space instead.
  
  This code adds the missing surrogate handling, and (to fix both issues), also removes non-printable characters instead of replacing them with a space.
  
  Also moved the `changed` test after these changes, so that changes in non-printable characters do not cause unneeded redraws.
  
  BUG: 376813
  FIXED-IN: 5.15.1

TEST PLAN
  `kwrite /tmp/Test😣.txt` shows correct title. I also tested actual non-printable characters, such as 0x1A, and these are correctly omitted.

REPOSITORY
  R108 KWin

REVISION DETAIL
  https://phabricator.kde.org/D19052

AFFECTED FILES
  client.cpp

To: cfeck, #kwin
Cc: kwin, jraleigh, GB_2, mkulinski, ragreen, jackyalcine, Pitel, iodelay, bwowk, ZrenBot, ngraham, alexeymin, lesliezhai, ali-mohamed, hardening, jensreuterberg, abetts, sebas, apol, mart