Enable hover effects by default

This makes it so we don't have to set `hoverEnabled: true` in Qt Quick Controls

src/platformtheme/khintssettings.cpp

@@ -143,7 +143,11 @@ KHintsSettings::KHintsSettings(KSharedConfig::Ptr kdeglobals)
     m_hints[QPlatformTheme::DialogButtonBoxButtonsHaveIcons] = readConfigValue(cg, QStringLiteral("ShowIconsOnPushButtons"), true);
     m_hints[QPlatformTheme::UseFullScreenForPopupMenu] = true;
     m_hints[QPlatformTheme::KeyboardScheme] = QPlatformTheme::KdeKeyboardScheme;
-    m_hints[QPlatformTheme::UiEffects] = readConfigValue(cg, QStringLiteral("GraphicEffectsLevel"), 0) != 0 ? QPlatformTheme::GeneralUiEffect : 0;
+
+    int uiEffectsFlags = readConfigValue(cg, QStringLiteral("GraphicEffectsLevel"), 0) != 0 ? QPlatformTheme::GeneralUiEffect : 0;
+    uiEffectsFlags |= QPlatformTheme::HoverEffect;
+    m_hints[QPlatformTheme::UiEffects] = uiEffectsFlags;
+
     m_hints[QPlatformTheme::IconPixmapSizes] = QVariant::fromValue(QList<int>() << 512 << 256 << 128 << 64 << 32 << 22 << 16 << 8);
 
     m_hints[QPlatformTheme::WheelScrollLines] = readConfigValue(cg, QStringLiteral("WheelScrollLines"), 3);

tests/HoverEffectTest.qml

+/*
+ * SPDX-FileCopyrightText: 2020 Noah Davis <noahadvs@gmail.com>
+ * SPDX-License-Identifier: LGPL-2.0-or-later
+ */
+
+import QtQuick 2.12
+import QtQuick.Templates 2.12
+
+ApplicationWindow {
+    width: control.implicitWidth + gridUnit*4
+    height: control.implicitHeight + gridUnit*4
+    color: "white"
+
+    property int gridUnit: fontMetrics.height
+
+    FontMetrics {
+        id: fontMetrics
+    }
+
+    Button {
+        id: control
+
+        implicitWidth: Math.round(Math.max(implicitContentWidth + leftPadding + rightPadding))
+        implicitHeight: Math.round(Math.max(implicitContentHeight + topPadding + bottomPadding))
+
+        anchors.centerIn: parent
+
+        padding: 8
+
+        // This is the default behavior, I'm just making it explicit.
+        hoverEnabled: Qt.styleHints.useHoverEffects
+
+        text: "There should be a red outline hover effect on this button"
+
+        contentItem: Label {
+            text: control.text
+            font: control.font
+            color: "white"
+        }
+
+        background: Rectangle {
+            radius: 3
+            color: "black"
+            border.color: "red"
+            // When the useHoverEffects style hint is true, the hover effect should work
+            border.width: control.hovered ? 4 : 0
+        }
+    }
+}