Correct location for palette paths

Palettes shouldn't be installed into a systemwide path, as they might be removed/overwritten when updating/uninstalling Inkscape and also interfere with the system's package/installation management.

They should be placed instead in the user's configuration/resource directory.

The same applies for OS X, but I don't know enough about OS filesystem layout and Inkscape's paths on OS X to make the appropriate change here.