Load non-system fonts from additional directories

This lays the foundation to load fonts from custom locations (i.e. lets Inkscape use fonts not installed on the system).

Feedback welcome!

**Implemented so far** * [x] Load fonts from Inkscape datadir, i.e. `share/inkscape/fonts` (can be switched off via preference, see "Text" tool) * [x] Load fonts from user configuration folder, i.e. `/inkscape/fonts` (can be switched off via preference, see "Text" tool) * [x] Load fonts from custom locations specified by the user * [ ] Load fonts from the location of the currently loaded SVG document