Install more screensavers on Mate desktop

I switched from Gnome3 to Mate desktop and noticed that I'm able to use only few sreensavers. If you're using Mate (it's default for Linux Mint) you might experience the same problem.

There is a workaround how to use much more screensavers with Mate as usually able to do with Gnome.

Install packages with additional screensavers:

apt-get install xscreensaver-data-extra xscreensaver-gl-extra

Go to /usr/share/applications/screensavers directory:

cd /usr/share/applications/screensavers

There are located number of .desktop files. You should edit them by replacing the line

OnlyShowIn=GNOME;

with

OnlyShowIn=GNOME;MATE;
```.

Obviously it's a routine to change them all manually. So use `sed` tool to edit all files in once:

```sh
find . -name '*.desktop' | xargs sed -i 's/OnlyShowIn=GNOME;/OnlyShowIn=GNOME;MATE;/'

Now you are able to use more than hundred screensavers on Mate!