LibGUI: GTabWidget can now return active tab index

`GTabWidget` now allows the user/caller to get the currently active
tab widget, meaning that actions that are applied globally (such as
an 'Apply' or 'OK' button) can now react to specific tabs etc.

Libraries/LibGUI/GTabWidget.cpp

@@ -208,3 +208,14 @@ void GTabWidget::set_tab_position(TabPosition tab_position)
         m_active_widget->set_relative_rect(child_rect_for_size(size()));
     update();
 }
+
+int GTabWidget::get_active_tab() const
+{
+    for(int i = 0; i < m_tabs.size(); i++)
+    {
+        if(m_tabs.at(i).widget == m_active_widget)
+            return i;
+    }
+
+    return -1;
+}

Libraries/LibGUI/GTabWidget.h

@@ -15,6 +15,7 @@ public:
 
     TabPosition tab_position() const { return m_tab_position; }
     void set_tab_position(TabPosition);
+    int get_active_tab() const;
 
     GWidget* active_widget() const { return m_active_widget; }
     void set_active_widget(GWidget*);