Remove self pointer from FxExpression::EvalExpression()

- Every place that calls this now (which are all in thingdef_parse.cpp)
  passed NULL for the self pointer, so it was superfluous to have around.
Removed some EvalExpression implementations

- Removed EvalExpression() implementations from some subclasses of
  FxExpression that don't make any sense in the context of evaluating a
  constant value.
Delete default expressions for action definitions

- After StateParams was removed in commit 4a1fbdb, the expressions for
  default paramaters that ParseActionDef() created went nowhere and were
  lost. Properly throw them away by deleting them as soon as they've been
  parsed.
Add degree variants of FLOP operations

- Added versions of the trig operations supported by FLOP that can work
  with degrees directly instead of radians.
- Reorder FLOPs into more sensible groupings.
Use new degree FLOPs for DECORATE expressions.

- Now that the VM supports degrees, use those instructions when emitting
  code for cos and sin DECORATE expressions. (This looks like it also
  fixed a bug where sqrt also did the degree to radian conversion.)