Custom predefined filters

The goal is to add new predefined filters with parameters (such as the existing Drop Shadow filter), by using the internal extensions system. I wouldn't be a full high level filter editor (such as https://blueprints.launchpad.net/inkscape/+spec/non-advanced-filters-ui), but just an UI help for creating custom filters based on predefined sets of primitives.
It could also be useful if we wish to reduce the number of predefined filters by grouping them by filter type.

Examples can be tested in the trunk, Filters>Experimental menu.