Okular Annotation: add line start/end style config only for Straight Line tool

Summary:
“Inner Color” configuration of Polygon tool was overlapping with the line start/end styles intended for only Straight Line tool. Fix it.

CCBUG: 381629

Test Plan:
1. Configure annotations
2. Create/Edit Polygon tool
3. Observe that no Line Start/End styles are visible
4. Create/Edit Straight Line tool
5. Observe that line start/end styles can be configured

Reviewers: #okular, tobiasdeiminger

Reviewed By: tobiasdeiminger

Subscribers: tobiasdeiminger, okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D21332