GafferUI : Fix crashes during shutdown

All these signals get connected to by Python functions, so will contain slots with a `PyObject *` in them. But Python shuts down _before_ static destructors are run, after which we can't delete the PyObject. The solution is one we've used elsewhere : just "leak" the signal and its connected slots. It's intended to live for the entire lifetime of the app anyway, so it doesn't matter if it's not cleaned up during shutdown.

Changes.md

@@ -21,6 +21,7 @@ Fixes
 - ImageGadget :
   - Fixed loading of 2 channel images [^1].
   - Fixed error message to include filename [^1].
+- GUI : Fixed potential crashes during shutdown [^1].
 
 API
 ---

src/GafferUI/PlugAdder.cpp

@@ -203,14 +203,14 @@ void PlugAdder::updateDragEndPoint( const Imath::V3f position, const Imath::V3f
 
 PlugAdder::PlugMenuSignal &PlugAdder::plugMenuSignal()
 {
-	static PlugMenuSignal s;
-	return s;
+	static PlugMenuSignal *s = new PlugMenuSignal;
+	return *s;
 }
 
 PlugAdder::MenuSignal &PlugAdder::menuSignal()
 {
-	static MenuSignal s;
-	return s;
+	static MenuSignal *s = new MenuSignal;
+	return *s;
 }
 
 void PlugAdder::renderLayer( Layer layer, const Style *style, RenderReason reason ) const

src/GafferUI/Pointer.cpp

@@ -146,6 +146,6 @@ void Pointer::registerPointer( const std::string &name, ConstPointerPtr pointer
 
 Pointer::ChangedSignal &Pointer::changedSignal()
 {
-	static ChangedSignal s;
-	return s;
+	static ChangedSignal *s = new ChangedSignal;
+	return *s;
 }