Okular Annotation: add support for line start style for Straight Line tool

Summary: Similar to the line ending style, add support for line start style for the Straight Line annotation tool

Test Plan:

1. Go to Configure annotations
2. Create (or edit existing) Straight Line tool
3. Set the ‘Line Start’ option on Style and Apply
4. Use the Straight Line tool to draw a line and check the line starting style.

Reviewers: Okular, tobiasdeiminger

Reviewed By: tobiasdeiminger

Subscribers: ngraham, tobiasdeiminger, okular-devel

Tags: Okular

Differential Revision: https://phabricator.kde.org/D21238