[BUGFIX] Page id detection does not work in BE in some cases

Page id detection does not work in Backend when trying to insert a new content element after the existing content element. In such case TYPO3 sends $GET['edit'] in a way that flux does not understand (see the attached screenshot). This is a special case in TYPO3 when the id is negative and equals to the uid of the content element after which the new element should be inserted. Special handling is needed for this case.

Currently this breaks BE layout in some cases, makes column selector wrong and inserts records into the wrong column.
![Correct columns](https://cloud.githubusercontent.com/assets/306133/2989505/bd28a88c-dc64-11e3-9570-d04acba266fe.png)
![Incorrect columns](https://cloud.githubusercontent.com/assets/306133/2989507/bd5b03ea-dc64-11e3-945b-3626601cebb8.png)
![Submitted data](https://cloud.githubusercontent.com/assets/306133/2989506/bd566c7c-dc64-11e3-9f65-311b27418bd9.png)