Use utils_get_uri_file_prefix() as file URI prefix

Use utils_get_uri_file_prefix() as file URI prefix

utils_get_uri_file_prefix() gives "file:///" for Windows and
"file://" for all other platforms. So we don't need "g_path_skip_root()"
any longer.
Using "g_path_skip_root()" removed the drive letter from the URI which
worked only as long as the file to be opened was on drive C: (or
whatever drive Windows considers as the default). But since local file
URIs including the drive letter are supported on Windows, we should use
it, so opening files on other drives works as well.

Fixes #1018.