tree-wide: rename draw_special_char to special_glyph

tree-wide: rename draw_special_char to special_glyph

That function doesn't draw anything on it's own, just returns a string, which
sometimes is more than one character. Also remove "DRAW_" prefix from character
names, TREE_* and ARROW and BLACK_CIRCLE are unambigous on their own, don't
draw anything, and are always used as an argument to special_glyph().

Rename "DASH" to "MDASH", as there's more than one type of dash.