libdeng1: Added a couple of new 2D vector box methods

To keep things a little tidier when updating using libdeng2's
de::VectorN<> classes.
Partition: Added intersection; intercept; isParallelTo

Derived from their libdeng1 counterparts.
Refactor|Vertex: Vertex origins are now defined as de::Vector2d

Also implemented a mechanism for updating the origin coords and added
de::Observers based audience for change notifications.
Refactor: Updated all map data classes wrt changes to Vertex
Refactor|BSP Builder: Updated wrt changes to Vertex
Refactor|Map Renderer: Updated Bias lighting wrt changes to Vertex

Also fixed a bug in the Bias lighting editor which transposed the
axes of the viewer origin in SBE_GetHand().
Refactor|Map Renderer: Updated FakeRadio wrt changes to Vertex

Also cleaned up some of the math, taking advantage of de::VectorN<>.
Refactor|Map Renderer: Miscellaneous updates wrt changes to Vertex
Refactor|Polyobj: Updated Polyobj wrt changes to Vertex
LineSightTest: Added default values for bottom/top slope and flags
Refactor: Updated PIT_AddLineIntercepts wrt changes to Vertex

The myriad problems with this function are now rather self evident.
It needs rewriting. However it would be better to ditch divline_t
and implement a fixed precision variant of de::Partition for this.
Refactor: Updated MPE_LineCreate wrt changes to Vertex
libdeng2|Vector: Added more methods, explicit Euclidean/homogeneous conversion
libdeng2|Vector: Added missing Vector2<> operator /