kcms/fonts: show warning when user tries to set a non-integer font size

It triggers https://bugreports.qt.io/browse/QTBUG-92006, and we need to
warn users about that.

CCBUG: 432372

kcms/fonts/package/contents/ui/main.qml

@@ -64,6 +64,29 @@ KCM.SimpleKCM {
             actions: [ kscreenAction ]
         }
 
+        Kirigami.InlineMessage {
+            id: fractionalFontSizeWarning
+            Layout.fillWidth: true
+            showCloseButton: true
+            type: Kirigami.MessageType.Warning
+            text: i18n("Decimal font sizes can cause text layout problems in some applications. Consider using only integer font sizes.")
+            // Specifically, it will trigger https://bugreports.qt.io/browse/QTBUG-92006,
+            // but we don't mention that in the message because that would be too technical
+            // for users.
+
+            Connections {
+                target: kcm
+                function onFontsHaveChanged() {
+                    fractionalFontSizeWarning.visible =
+                    !Number.isInteger(generalFontWidget.font.pointSize)
+                    || !Number.isInteger(fixedWidthFontWidget.font.pointSize)
+                    || !Number.isInteger(smallFontWidget.font.pointSize)
+                    || !Number.isInteger(toolbarFontWidget.font.pointSize)
+                    || !Number.isInteger(menuFontWidget.font.pointSize)
+                }
+            }
+        }
+
         Kirigami.InlineMessage {
             id: dpiTwiddledMessage
             Layout.fillWidth: true