xwl: No need to delete the source immediately

Prefer replacing then deleting the selection to the other way around.
This way less events need to be reported and the clipboard manager
doesn't get confused.

(cherry picked from commit 4150c1f0)

xwl/selection.cpp

@@ -167,9 +167,11 @@ void Selection::registerXfixes()
 
 void Selection::setWlSource(WlSource *source)
 {
-    delete m_waylandSource;
+    if (m_waylandSource) {
+        m_waylandSource->deleteLater();
+        m_waylandSource = nullptr;
+    }
     delete m_xSource;
-    m_waylandSource = nullptr;
     m_xSource = nullptr;
     if (source) {
         m_waylandSource = source;
@@ -179,10 +181,7 @@ void Selection::setWlSource(WlSource *source)
 
 void Selection::createX11Source(xcb_xfixes_selection_notify_event_t *event)
 {
-    delete m_waylandSource;
-    delete m_xSource;
-    m_waylandSource = nullptr;
-    m_xSource = nullptr;
+    setWlSource(nullptr);
     if (!event || event->owner == XCB_WINDOW_NONE) {
         return;
     }