Summary:
Use `LineAnnotPainter` to generate accurate icons.

Looks like this:
{F6852054}

Depends on D21248

Test Plan:
- open "Edit annotation tool" for straight line and check combo box items for start and stop style
- test on HiDPI

Reviewers: #okular, sander

Reviewed By: sander

Subscribers: davidhurka, knambiar, ngraham, okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D21416