Reworked PasteItem tests (use ftw.testbrowser instead of OGBrowser).
Adjust available expression for paste action.

Use `object` instead of `folder` to call the `is_pasting_allowed`, because it's
not guaranteed that all folder allways returns the current context.
Disable paste action for mails.
Merge pull request #665 from 4teamwork/pg_disable_pasting_for_mails

Disable pasting action for mails