Views integration improvements

This most likely will be postponed, but I want to submit it for anyone willing to improve Views integration. This is based on my experience of working with Views. If anyone has more ideas, feel free to share!

1. Don't declare table joins in hook_views_data(). These are "hidden" JOINs, which happen automatically which means user does't control it. This leads to ugly workarounds such as setting "gallery id > 0" filter to emulate INNER JOIN. Note that it's possible to set up INNER JOIN in the table declaration, but it then removes possibility of using LEFT JOIN
2. Use relationships where possible. Relationships provide user-controlled joins. In example above, user could just check "require this relationship".
3. Views relationships also can have UI to select their settings, which means it's possible to support Node Gallery relationships better