More PKGBUILD fixes

Hi,

OK, that first pull-request taught me you guys work fast. Great. Hopefully, this pull-request will go some way to fixing up `mate-extras`.

  * Bumped `pkgrel` in `mate-file-manager-open-terminal` to ensure it is now listed in `mate-extras`.
  * Fixed typo in `mate-file-manager-gksu` to ensure it is now listed in `mate-extras`.
  * Fixed typo in `mate-file-manager-image-converter` to ensure it is now listed in `mate-extras`.
  * Fixed typo in `mate-system-tools` to ensure it is now listed in `mate-extras`.
  * Added a description to `mate-netspeed` and bumped the `pkgrel` to ensure it is now listed in `mate-extras`.
  * Removed `dconf` from `mate-terminal` requirements.

I'll see what else I can work on next week. Enjoy the weekend.

Regards, Martin.