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/SetLabelCommand.cpp

@@ -66,8 +66,8 @@ bool SetLabelCommand::Apply(const CommandContext & context)
    // this code could be put in subroutines/reduced.
 
    //wxString mode = GetString(wxT("Type"));
-   AudacityProject * p = context.GetProject();
-   TrackList *tracks = context.GetProject()->GetTracks();
+   AudacityProject * p = &context.project;
+   TrackList *tracks = context.project.GetTracks();
    LabelStruct * pLabel = NULL;
    int i=0;
    int nn=0;