collectionview: Show correct tooltip

Prior to this patch Korganizer showed tooltips for the wrong
collections. Simplified this code to generate the tooltip string on
demand as well, this also fixes the issue.

src/akonadicollectionview.cpp

@@ -335,6 +335,16 @@ protected:
         }
         return true;
     }
+
+    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override
+    {
+        if (role == Qt::ToolTipRole) {
+            const Akonadi::Collection col = CalendarSupport::collectionFromIndex(index);
+            return CalendarSupport::toolTipString(col);
+        }
+
+        return QSortFilterProxyModel::data(index, role);
+    };
 };
 
 class CalendarDelegateModel : public QSortFilterProxyModel

src/views/collectionview/calendardelegate.cpp

@@ -54,6 +54,8 @@ static QStyle *style(const QStyleOptionViewItem &option)
 
 static QStyleOptionButton buttonOpt(const QStyleOptionViewItem &opt, const QPixmap &pixmap, const QModelIndex &index, int pos = 1)
 {
+    Q_UNUSED(index);
+
     QStyleOptionButton option;
     option.icon = pixmap;
     QRect r = opt.rect;
@@ -61,11 +63,6 @@ static QStyleOptionButton buttonOpt(const QStyleOptionViewItem &opt, const QPixm
     option.rect = enableButtonRect(r, pos);
     option.state = QStyle::State_Active | QStyle::State_Enabled;
     option.iconSize = QSize(h, h);
-    auto *w = const_cast<QWidget *>(opt.widget);
-    if (w) {
-        const Akonadi::Collection col = CalendarSupport::collectionFromIndex(index);
-        w->setToolTip(CalendarSupport::toolTipString(col));
-    }
     return option;
 }