Change ajax endpoint multifile

I found out that `/src/Backend/Ajax.php` didn't work. The endpoints were changed in [this commit](https://github.com/forkcms/forkcms/commit/f0b2a996912a66a10e3fb8433ca5bc5e311a1867) of Fork CMS. It's now `/backend/ajax`.
Fix looking for qqFileUploader in same namespace

qqFileUploader needed a backslash in order to work. Else it will search in the same namespace (Ajax) for the class. I got this error without backslash: `Error: Class 'Backend\Modules\Routes\Ajax\qqFileUploader' not found`.
Merge pull request #55 from jessedobbelaere/fix-multifileupload-bugs

Fix multifileupload bugs