Expressions

- Filter processing short-circuits if there is no filter.
- Variant code was not only incomplete, but very broken.  It should not have
  been used.  It still doesn't handle dates and durations.
- Converted all logical and relational Variant operators to return Boolean
  values.
- Stack size is now checked for every operator.
- All operators implemented (without any advanced special case handling) except
  %, ~ and !~.
- Added lazy DOM expansion at the last possible moment.
- Implemented Expression::create_variant to create appropriate Variant instances
  based on categorized and inferred type.
- Removed Variant math functions.  No point.
- Debug code left in place for now.

src/Expression.h

@@ -32,6 +32,7 @@
 #include <stack>
 #include <Arguments.h>
 #include <Task.h>
+#include <Variant.h>
 
 class Expression
 {
@@ -51,6 +52,7 @@ private:
   void expand_tokens ();
   void postfix ();
 
+  void create_variant (Variant&, const std::string&, const std::string&);
   bool is_new_style ();
 
 private: