Refactored ElemValue

Checkpoint: beginning of a huge rework of the various "value" classes,
to make a bit more sense.
Moved PropValue

Checkpoint
Moved TypedValue and Context

Checkpoint.
Moved PropAndVal, and fixed TypedValue and Context

Checkpoint.
Introduced the notion that PTypeBuilder knows its PType

Checkpoint
Critical bugfix: QL syntax issue

Error in my QL parser was disallowing external-formatted links!
Release 0.3.15.2