Some minor PKGBUILD fixes.

Hi,

I've migrated from GNOME3 to MATE 1.6 and noticed a few minor issues with the packages.

  * Removed `dconf` from `mate-file-archiver` requirements.
  * Added a description to `mate-netbook`.
  * Added `mate-screensaver` to `mate-extras` group.

This is an initial pull request to get a feel for how your guys work. Once I get a feel for how to contribute and what's expected I have some other PKGBUILD related stuff I'd like to contribute.

Regards, Martin.