There should be sensible defaults for filters wherever possible

[bhaddow]

Often adding a filter fails, because there are no default arguments passed to the filter function. For example many functions require source and target language and this could be set globally at a corpus level. Because it isn't, this requires fiddling with the dropdowns for many of the filters, and reading obscure error messages.

[jelmervdl]

Related to #45.

Filters do support static default arguments already, but it looks like I never bothered setting good defaults 🤦