Add negate option for block visibility by content type

Problem/Motivation

It is currently not possible to negate the content type selection in the block visibility settings. In other words if I need to hide a block on content type A, but keep it on type B, C and any other types, including those not yet created - I can't do so.

Proposed resolution

Add a "show/hide" option to the Content type visibility setting tab, similar to the one found on the Pages visibility setting tab.

Remaining tasks

- discuss
- provide a patch
- test