Improved highlighting of polylines and symbols

Summary: This change improves the highlighting of polylines and symbols several ways.

Test Plan:
Check that the highlighting of polylines and symbols provides a better
visual feedback when hovering over selected as well as unselected items.

Reviewers: lisbethb

Reviewed By: lisbethb

Projects: #diana_new_drawing_tool

Differential Revision: https://phab.met.no/D118