Refactor getDOM to use a pointer for the optional context

It's possible to call getDOM without a contextual task.  Previously,
this was done by referencing a "dummy" task which necessitated a way to
distinguish such dummy tasks.  This switches to using a pointer and
treating the NULL value as meaning there is no context.

Note that this cannot use `std::optional<&Task>`, as optional does not
support reference types.

src/columns/ColProject.cpp

@@ -36,7 +36,7 @@
 #include <utf8.h>
 #include <util.h>
 
-extern Task& contextTask;
+extern Task* contextTask;
 
 ////////////////////////////////////////////////////////////////////////////////
 ColumnProject::ColumnProject ()
@@ -121,7 +121,7 @@ void ColumnProject::modify (Task& task, const std::string& value)
     {
       Eval e;
       e.addSource (domSource);
-      contextTask = task;
+      contextTask = &task;
 
       Variant v;
       e.evaluateInfixExpression (value, v);