Is that a way to clear all filter, such as clearAllFilters()?
Nope, there is no a separate API for such task.
You can use combination of dtable.eachColumn and getFilter api to get filters one by one and set empty values to them.
OK~
I tried to check whether or not a filter was set by getFilter() but this causes the debugger to kick in – so how am I supposed to check for the existence of a filter?
In the default Webix version getFilter will work smoothly, but the debug version will throw this error anyway. There’s no way to avoid this.
would be nice to have a native clearAllFilter() function… that would be the way to avoid the debug version messages 
here’s my working implementation…
datatable.eachColumn( function(pCol) { var f = this.getFilter(pCol); if (f) if (f.value) f.value = “”; });