Rename the included theme to Default, with 4 variants:
light, dark, hc, hc-dark. This replaces Adwaita,
Adwaita:dark, HighContrast and HighContrastInverse.
We still make the themes available under these names,
and we still set up Adwaita-dark and HighContrastInverse
as the dark variants of Adwaita and HighContrast.

The unification of the theme variants under Default
is not quite perfect; it would be nice to merge
the assets/ and assets-hc/ subdirectories and render
all assets from a single svg file.