D16908: [decorations] Emit DecorationSettings::fontChanged signal

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REVISION SUMMARY
  No one emits DecorationSettings::fontChanged signal, so if you change
  the window title font, then titlebars might look differently after reboot.
  
  Currently, there are two places where we can emit that signal:
  
  - in KDecoration library itself;
  - and in KWin.
  
  Because we would need to listen for a Plasma specific D-Bus signal, the
  latter option is more preferred.
  
  Surprisingly, KWin's implementation of DecorationSettingsPrivate already
  reacts to refreshFonts D-Bus signal (even though indirectly), so all
  what we have to do is get the current window title font in
  SettingsImpl::readSettings, and if it's different from the previous one,
  emit DecorationSettings::fontChanged signal.
  
  BUG: 400980
  FIXED-IN: 5.15.0

TEST PLAN
  - Increased the size of the window title font, titlebars got bigger;
  - Decreased the font size; titlebars got smaller.

REPOSITORY
  R108 KWin

BRANCH
  emit-fontchanged-signal

REVISION DETAIL
  https://phabricator.kde.org/D16908

AFFECTED FILES
  decorations/settings.cpp
  decorations/settings.h

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