Add line annotation ending arrows for non PDF documents

Summary:
This implements drawing the various line ending styles for 2 point lines in non-PDF documents.

Looks like this:
{F6833437}

CCBUG: 381629

Test Plan:
- open a *.txt document
- draw line annotations with different arrow styles: Square, Diamond, OpenArrow, ClosedArrow, ROpenArrow, RClosedArrow, Butt, Slash, Circle
- ... as start and as end style (start needs D21238 to be configurable in GUI)
- ... filled or not (fill color can't be configured in GUI yet)
- ... with different leader line settings
- ... at various angles
- rotate and scale page
- leader line with setting 100 gives 100 pixel line at 100% zoom (use kruler to verify)

Reviewers: #okular, aacid, sander

Reviewed By: sander

Subscribers: ngraham, sander, knambiar, okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D21248