URIs and Filenames

Inkscape's handling of URIs and Filenames is a complete mess!

What some variable names claim to be URIs are actually filenames and visa-versa.
There is a lot of code duplication. (e.g. src/io/dir-utils.cpp and src/xml/rebase-hrefs.cpp).

Done

Put all our own URI and Filename handling code in src/io (or at least list in README in that directory).
Convert all routines to use:
- std::string (byte string) for file names and paths.
- Glib::ustring (unicode string) for URIs
Rely on Glib functions where possible. Glib functions handle many of the WIN32/LINUX/MAC issues automatically.
- Use Gio::File class.
- Use Glib utility and conversion functions (see below).
Remove 'sodipodi-absref' as this leaks user directory information.
Clarify whether/when to use these classes (most stuff in Inkscape::IO should probably be removed, Tav):
- Inkscape::URI
- Inkscape::IO::UriInputStream
  - Why does UriInputStream not decode data: URIs?

URIs should always use '/'. You cannot use Glib filename functions to handle URIs as they will use '/' or '\' depending on operating system.
Use relative paths (URIs!) where possible as this allows moving documents with resources together.

bool Glib::path_is_absolute (const std::string& filename): Returns true if the given filename is an absolute file name, i.e. it contains a full path from the root directory such as "/usr/local" on UNIX or "C:\\windows" on Windows systems. More...

std::string Glib::path_skip_root (const std::string& filename): Returns the remaining part of filename after the root component, i.e. after the "/" on UNIX or "C:\\" on Windows. More...

std::string Glib::path_get_basename (const std::string& filename): Gets the name of the file without any leading directory components. More...

std::string Glib::path_get_dirname (const std::string& filename): Gets the directory components of a file name. More...

std::string Glib::build_filename (const Glib::ArrayHandle< std::string >& elements): Creates a filename from a series of elements using the correct separator for filenames. More...

std::string Glib::build_filename (const std::string& elem1, const std::string& elem2): Creates a filename from two elements using the correct separator for filenames. More...

std::string Glib::build_filename (const std::string& elem1, const std::string& elem2, const std::string& elem3): Creates a filename from three elements using the correct separator for filenames. More...

etc.

Glib::ustring Glib::filename_to_utf8 (const std::string& opsys_string): Converts a string which is in the encoding used for filenames into a UTF-8 string.

std::string Glib::filename_from_utf8 (const Glib::ustring& utf8_string): Converts a string from UTF-8 to the encoding used for filenames.

std::string Glib::filename_from_uri (const Glib::ustring& uri, Glib::ustring& hostname): Converts an escaped UTF-8 encoded URI to a local filename in the encoding used for filenames.

std::string Glib::filename_from_uri (const Glib::ustring& uri): Converts an escaped UTF-8 encoded URI to a local filename in the encoding used for filenames.

Glib::ustring Glib::filename_to_uri (const std::string& filename, const Glib::ustring& hostname): Converts an absolute filename to an escaped UTF-8 encoded URI.

Glib::ustring Glib::filename_to_uri (const std::string& filename): Converts an absolute filename to an escaped UTF-8 encoded URI.

Glib::ustring Glib::filename_display_basename (const std::string& filename): Returns the display basename for the particular filename, guaranteed to be valid UTF-8.

Glib::ustring Glib::filename_display_name (const std::string& filename): Converts a filename into a valid UTF-8 string.