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);

src/columns/ColRecur.cpp

@@ -36,7 +36,7 @@
 #include <format.h>
 #include <utf8.h>
 
-extern Task& contextTask;
+extern Task* contextTask;
 
 ////////////////////////////////////////////////////////////////////////////////
 ColumnRecur::ColumnRecur ()
@@ -108,7 +108,7 @@ void ColumnRecur::modify (Task& task, const std::string& value)
   {
     Eval e;
     e.addSource (domSource);
-    contextTask = task;
+    contextTask = &task;
     e.evaluateInfixExpression (value, evaluatedValue);
   }
 

src/columns/ColTags.cpp

@@ -36,7 +36,7 @@
 #include <utf8.h>
 #include <main.h>
 
-extern Task& contextTask;
+extern Task* contextTask;
 
 ////////////////////////////////////////////////////////////////////////////////
 ColumnTags::ColumnTags ()
@@ -162,7 +162,7 @@ void ColumnTags::modify (Task& task, const std::string& value)
   {
     Eval e;
     e.addSource (domSource);
-    contextTask = task;
+    contextTask = &task;
 
     Variant v;
     e.evaluateInfixExpression (value, v);

src/columns/ColTypeDate.cpp

@@ -34,7 +34,7 @@
 #include <Filter.h>
 #include <format.h>
 
-extern Task& contextTask;
+extern Task* contextTask;
 
 ////////////////////////////////////////////////////////////////////////////////
 ColumnTypeDate::ColumnTypeDate ()
@@ -213,7 +213,7 @@ void ColumnTypeDate::modify (Task& task, const std::string& value)
   {
     Eval e;
     e.addSource (domSource);
-    contextTask = task;
+    contextTask = &task;
     e.evaluateInfixExpression (value, evaluatedValue);
   }
 

src/columns/ColTypeDuration.cpp

@@ -32,7 +32,7 @@
 #include <Filter.h>
 #include <format.h>
 
-extern Task& contextTask;
+extern Task* contextTask;
 
 ////////////////////////////////////////////////////////////////////////////////
 ColumnTypeDuration::ColumnTypeDuration ()
@@ -55,7 +55,7 @@ void ColumnTypeDuration::modify (Task& task, const std::string& value)
   {
     Eval e;
     e.addSource (domSource);
-    contextTask = task;
+    contextTask = &task;
     e.evaluateInfixExpression (value, evaluatedValue);
   }
 

src/columns/ColTypeNumeric.cpp

@@ -32,7 +32,7 @@
 #include <Filter.h>
 #include <format.h>
 
-extern Task& contextTask;
+extern Task* contextTask;
 
 ////////////////////////////////////////////////////////////////////////////////
 ColumnTypeNumeric::ColumnTypeNumeric ()
@@ -55,7 +55,7 @@ void ColumnTypeNumeric::modify (Task& task, const std::string& value)
   {
     Eval e;
     e.addSource (domSource);
-    contextTask = task;
+    contextTask = &task;
     e.evaluateInfixExpression (value, evaluatedValue);
   }
 

src/columns/ColTypeString.cpp

@@ -34,7 +34,7 @@
 
 #define STRING_INVALID_MOD           "The '{1}' attribute does not allow a value of '{2}'."
 
-extern Task& contextTask;
+extern Task* contextTask;
 
 ////////////////////////////////////////////////////////////////////////////////
 ColumnTypeString::ColumnTypeString ()
@@ -67,7 +67,7 @@ void ColumnTypeString::modify (Task& task, const std::string& value)
   {
     Eval e;
     e.addSource (domSource);
-    contextTask = task;
+    contextTask = &task;
 
     Variant v;
     e.evaluateInfixExpression (value, v);