Lock the filament feeder in place

When I pushed the filament manually, I had to prevent the feeder from popping out of its attachment slots.

I designed this thing to make my life easier. Interestingly, it is also one quite popular object on thingiverse, so there must be a lot of people annoyed with this attachment (and because of failure with the hobbed bolt).

Actually I almost never had to push the filament manually anymore, but this is due to my new and own filament feeder design.