Double clicking on a tab will make it float

Also really required for Wayland, where we don't another way to
float a single tab, as the float button affects all of them.

src/private/widgets/TabBarWidget.cpp

@@ -111,6 +111,11 @@ void TabBarWidget::mouseMoveEvent(QMouseEvent *e)
     }
 }
 
+void TabBarWidget::mouseDoubleClickEvent(QMouseEvent *e)
+{
+    TabBar::onMouseDoubleClick(e->pos());
+}
+
 bool TabBarWidget::dragCanStart(QPoint pressPos, QPoint pos) const
 {
     // Here we allow the user to re-order tabs instead of dragging them off.

src/private/widgets/TabBarWidget_p.h

@@ -47,6 +47,7 @@ protected:
     bool dragCanStart(QPoint pressPos, QPoint pos) const override;
     void mousePressEvent(QMouseEvent *) override;
     void mouseMoveEvent(QMouseEvent *e) override;
+    void mouseDoubleClickEvent(QMouseEvent *e) override;
 private:
     TabWidget *const m_tabWidget;
 };

src/private/widgets/TabWidget.cpp

@@ -115,6 +115,12 @@ void TabBar::onMousePress(QPoint localPos)
     m_lastPressedDockWidget = dockWidgetAt(localPos);
 }
 
+void TabBar::onMouseDoubleClick(QPoint localPos)
+{
+    if (DockWidgetBase *dw = dockWidgetAt(localPos))
+        dw->setFloating(true);
+}
+
 bool TabBar::hasSingleDockWidget() const
 {
     return numDockWidgets() == 1;

src/private/widgets/TabWidget_p.h

@@ -67,6 +67,7 @@ public:
     FloatingWindow *detachTab(DockWidgetBase *dockWidget);
 
     void onMousePress(QPoint localPos);
+    void onMouseDoubleClick(QPoint localPos);
 
     ///@brief returns whether there's only 1 tab
     bool hasSingleDockWidget() const;