D20405: Make preview(dashed rectangle) of rectangle annotation colorful

[Ahmed Bilal]

ahmedbilal added a reviewer: sander.

REPOSITORY
  R223 Okular

BRANCH
  make_preview_rect_colorful

REVISION DETAIL
  https://phabricator.kde.org/D20405

To: ahmedbilal, ngraham, sander
Cc: ngraham, sander, okular-devel, joaonetto, tfella, darcyshen, aacid