umbrello/umlwidgets/associationwidget.{h,cpp} followup to commit 136460f6

- At function linePathStartsAt add `const' qualifier.
- Add function linePathEndsAt in symmetry with linePathStartsAt.
- In associationwidget.cpp,
  - define preprocessor symbol PIXEL_TOLERANCE for use in functions
    linePathStartsAt/linePathEndsAt;
  - in functions setStereotype and mouseDoubleClickEvent replace call to
    uDebug() by DEBUG(DBG_SRC) for control from within Umbrello's Debug
    dock.

CCBUG: 447866