Flowline Points: internal representation changed to point vector (QPolygonF).
Bug Number: n/a
Release notes (y/n): no
For Release Nums: 6.7

The internal representation of Flow Line vertex coordinates has been recoded 
as a Poly-Line (point vector).  The current flow line graphics item AND user 
interface implementation doesn't yet support having more than two points for 
each Flow Line.  But Output Canvases created with this new implementation 
will be upward compatible with future versions which do support true 
multiple-point (>2) flow lines.

NOTE that no automatic conversion was implemented.  Flow lines in existing 
test/demo models WILL NEED TO BE FIXED or RECREATED.

This commit represents completion of task (3A) in this project planning 
document:

  Output Canvas / Flow Lines Development Plan / February 2015
  http://cadswes2.colorado.edu/~philw/2015/OutputCanvas/FlowLines/FlowLinesDevPlan-Feb2015.html
  Task 3: Flow Lines as "Poly-Lines" (two or more vertices) ...
  ... (3A) Change internal representation of Flow Line vertices

The initial provisional implementation of flow lines was explicitly limited 
to two-point line segments, encoded in four integer rwSettings:

  rwIntSetting*  _end1PosX;    // FLOWLINE_END1_POSX
  rwIntSetting*  _end1PosY;    // FLOWLINE_END1_POSY
  rwIntSetting*  _end2PosX;    // FLOWLINE_END2_POSX
  rwIntSetting*  _end2PosY;    // FLOWLINE_END2_POSY

Also removed were provisional "SNAP" rwSettings which were never actually 
developed.  (This was premature. We haven't actually figured out how we are 
going to attach flow lines vertices).

  rwTextSetting*  _end1Snap;   // FLOWLINE_END1_SNAP
  rwTextSetting*  _end2Snap;   // FLOWLINE_END2_SNAP

These have been removed, and replaced with a non-rwSetting QPolygonF field:

  QPolygonF _polyLine; (QPolygonF is a subclass of QVector<QPointF>).

An implicit change is that these coordinates are represented with floating 
point (double) values instead of integers.  Previously, integers were used 
only because flow line vertex coordinates were represented as rwIntSettings.
This resulted in one-off value loss because of certain numeric computations 
-- you could sometimes actually see the line jump by one pixel.  (This still 
occurs with teacups and canvas text items).  Even though floating point 
values are now used internally, the serialization does seem to produce 
integer values.  Here is an example:

  <FlowLineInst>
    <FlowLinePoint inx="0" x="207" y="410" />
    <FlowLinePoint inx="1" x="33" y="476" />
    <SettingText>
    ... ... ...

Some preparation work was done to generalize OcanFlAnchorPntItem (flow 
line anchor point graphics objects) to be associated with any FlowLine 
vertex, rather than just one of two end-vertices.  See struct FlowLinePnt 
in QtUtils/OcanFlAnchorPntItem.

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