Remove CommandContext::GetProject() ...

... which called ::GetActiveProject(), but one purpose of the CommandContext
class was to eliminate many uses of that global function, because a
CommandContext must always be constructed from a reference to a project
(which was always in fact the active one), then passed around to where it is
needed!

Also, just use the project member directly -- because CommandContext was
intended as just a P.O.D (plain-old-data) structure.

This also eliminates a dependency of CommandContext.cpp on Project.cpp.

This is not enough by itself to break any dependency cycles.

src/commands/CommandHandler.h

@@ -25,7 +25,7 @@ class CommandContext;
 class CommandHandler
 {
    private:
-      std::unique_ptr<CommandContext> mCurrentContext;
+      std::unique_ptr<const CommandContext> mCurrentContext;
 
    public:
       CommandHandler();