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 preferable.
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.