Add option to extend scene sequences (#121)

Extending a scene sequence allows for more complex switching setups, but should also simplify some setups.
Sequences like "A -> B -> A -> C", which previously were only possible by creating a copy of A, can now be specified in a single entry.
To extend a sequence either select the sequence you want to modify and click the extend sequence button or simply double click the entry.

forms/advanced-scene-switcher.ui

@@ -2824,6 +2824,20 @@
            </property>
           </widget>
          </item>
+         <item>
+          <widget class="Line" name="line">
+           <property name="orientation">
+            <enum>Qt::Vertical</enum>
+           </property>
+          </widget>
+         </item>
+         <item>
+          <widget class="QPushButton" name="sequenceEdit">
+           <property name="text">
+            <string>AdvSceneSwitcher.sceneSequenceTab.extendEdit</string>
+           </property>
+          </widget>
+         </item>
          <item>
           <spacer name="horizontalSpacer_8">
            <property name="orientation">