Merge branch 'sev/more_crosshairs' into 'master'

A few additional crosshairs

This is more a merge suggestion than a request (sorry for the recent avalanche).

Some time ago I created a few crosshairs that I feel are missing in the default set:
simple traditional design, unobtrusive, high-contrast.

Now I've noticed that the new menu has an empty row in the crosshair settings.

Add one and one together and you get this branch.

![crosshairs](https://gitlab.com/uploads/xonotic/xonotic-data.pk3dir/14dc267bef/crosshairs.png)

See merge request !94