Add icons for line annotation arrow styles to combo box

Summary:
Use LineAnnotPainter to generate accurate icons.

Looks like this:

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