InteractivePageInterface has now something it really needs to be
implemented
Now if an error occures during the tag rendering process, the error
message and line is shown
Ajax paths have now to start with /ajax/ and if a requested file is not
found, it is logged as a warning
For now an InteractivePage has to be a StatefulPage
Helper class added for InteractivePages
Now the post parsing should now work better
This should prevent empty or nonexisting files to be uploaded