Fix for #6607: Implemented deletion of all questions

Question locking is currently WIP and does not work yet.
Fixed #6621: Hide loading mask through callback

There are now multiple loading masks allowed. Each will have a
minimum duration of 500ms to reduce flickering. Because I removed
the original `hideLoadMask` method, there could remain some masks
for their full duration. Please report these issues in order for me
to fix them.