[kcmkwin/kwindesktop] Make Slide effect configurable

Summary:
Even though the Slide effect has a KCM, it's not possible to configure
it [slide effect] from the virtual desktops KCM.

This change addresses the problem above.

### Before

{F5912774, layout=center, size=full}

### After

{F5912775, layout=center, size=full}

BUG: 395377

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13544

kcmkwin/kwindesktop/main.cpp

@@ -521,9 +521,16 @@ void KWinDesktopConfig::slotEffectSelectionChanged(int index)
     if (index != 0)
         enabled = true;
     m_ui->effectInfoButton->setEnabled(enabled);
-    // only cube has config dialog
+
-    if (index != 2)
+    switch (index) {
+    case 1: // Slide
+    case 2: // Cube Slide
+        enabled = true;
+        break;
+    default:
         enabled = false;
+        break;
+    }
     m_ui->effectConfigButton->setEnabled(enabled);
 }
 
@@ -610,6 +617,9 @@ void KWinDesktopConfig::slotConfigureEffectClicked()
 {
     QString effect;
     switch(m_ui->effectComboBox->currentIndex()) {
+    case 1:
+        effect = BuiltInEffects::nameForEffect(BuiltInEffect::Slide);
+        break;
     case 2:
         effect = BuiltInEffects::nameForEffect(BuiltInEffect::CubeSlide);
         break;