Pointer cleanup (#242)

- Made greater use of smart pointers, eliminating many instances of manual memory management
- Do not use pointers at all for many non-polymorphic classes
- Assorted other code cleanup

model/CMakeLists.txt

@@ -31,7 +31,7 @@ set (Model_sources
   FramePath.cpp
   FrameRect.cpp
   FrameRound.cpp
-  Handles.cpp
+  Handle.cpp
   Layout.cpp
   Markup.cpp
   Model.cpp