Fix x11 set_string_contents

[mcobzarenco]

The x11 clipboard does not seem to work on linux. It correctly returns the content, but it fails to set it.

I think there are two different problems:

1. In set_string_contents, getter.atoms is used instead of setter.atoms
2. "atoms.primary" is used instead of "atoms.clipboard" (the latter is the default in the original rust_clipboard. Changing it fixes the issue. Are there any reasons not to use atoms.clipboard?

Based on #4