Add missing cursors for Chromium-based apps

Summary: The Cursor Specification doesn't even mention these two, but Chromium-based apps search for them using a different name than ours, so it wouldn't hurt to symlink them.

Test Plan:
This can be tested on any webpage using either `cursor: grab` or `cursor: grabbing` CSS rule. But here's a page where you can easily test them:

http://elektronotdienst-nuernberg.de/bugs/cursor.html

Reviewers: #breeze, broulik, ngraham

Reviewed By: #breeze, broulik, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27424