Fully implement IContextMenu

Although all of this stuff is completely untested, and since no host on
Linux uses it we'll likely never know whether this implementation is
correct.

src/common/serialization/vst3/README.md

@@ -6,8 +6,8 @@ for more information on how the serialization works.
 
 The following interfaces are not yet implemented:
 
-- Every interface introduced after VST 3.1.0 with the exception of
-  `INoteExpressionController` which has already been implemented
+- Every interface introduced after VST 3.1.0, although most of the VST 3.5.0
+  interfaces have already been implemented
 - The [Presonus extensions](https://presonussoftware.com/en_US/developer),
   although most of these things seem to overlap with newer VST3 interfaces
 

src/common/serialization/vst3/context-menu/context-menu.h

@@ -150,7 +150,7 @@ class YaContextMenu : public Steinberg::Vst::IContextMenu {
     };
 
     virtual tresult PLUGIN_API
-    removeItem(const Item& item,
+    removeItem(const Steinberg::Vst::IContextMenuItem& item,
                Steinberg::Vst::IContextMenuTarget* target) override = 0;
 
     /**