universal-access: Add "animations" to desktop file keywords

In commit a825ded0 we added the option
to enable/disable animations in the desktop as an accessibility feature.

Adding the keyword makes it possible to find the Accessibility
settings while searching for "animations" in the shell or in
gnome-control-center.

Fixes #1620
8 jobs for universal-access-add-animations-to-desktop-file-keywords in 4 minutes and 57 seconds (queued for 7 seconds)