[decorations] Emit DecorationSettings::fontChanged signal

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 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.

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.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16908

decorations/settings.cpp

@@ -30,6 +30,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 #include <KConfigGroup>
 
+#include <QFontDatabase>
+
 namespace KWin
 {
 namespace Decoration
@@ -183,6 +185,11 @@ void SettingsImpl::readSettings()
         m_borderSize = size;
         emit decorationSettings()->borderSizeChanged(m_borderSize);
     }
+    const QFont font = QFontDatabase::systemFont(QFontDatabase::TitleFont);
+    if (font != m_font) {
+        m_font = font;
+        emit decorationSettings()->fontChanged(m_font);
+    }
 
     emit decorationSettings()->reconfigured();
 }

decorations/settings.h

@@ -49,6 +49,9 @@ public:
     QVector< KDecoration2::DecorationButtonType > decorationButtonsRight() const override {
         return m_rightButtons;
     }
+    QFont font() const override {
+        return m_font;
+    }
 
 private:
     void readSettings();
@@ -59,6 +62,7 @@ private:
     QVector< KDecoration2::DecorationButtonType > m_rightButtons;
     KDecoration2::BorderSize m_borderSize;
     bool m_closeDoubleClickMenu = false;
+    QFont m_font;
 };
 } // Decoration
 } // KWin