`_todo` glyph, proto accents, more…

*   `backslash` now defined in terms of `slash`.
*   `exclamdown` and `questiondown` now defined in terms of `exclam` and `question`, respectively.
*   `nbspace` now defined in terms of `space`.
*   Several, several primitive accented glyphs have been added by composing letters with marks. They are indicated with the notation `"fix" : "above"`, which currently doesn't do anything and is only an indicator for where the position of an accent hasn't been properly set yet.
*   A `_todo` glyph is added.
*   The splitter now generates a `_todo`-based glyph for any graphic character from Windows-1252 that has not been implemented yet.
*   The splitter now maps only the earliest name for a codepoint when reversing the name-to-codepoint mapping (which is not 1:1) in order to keep the result predictable.