Parses a string representing an accelerator. The format looks like "<Control>a" or "<Shift><Alt>F1" or "<Release>z" (the last one is for key release). The parser is fairly liberal and allows lower or upper case, and also abbreviations such as "<Ctl>" and "<Ctrl>". Key names are parsed using gdk_keyval_from_name(). For character keys the name is not the symbol, but the lowercase name, e.g. one would use "<Ctrl>minus" instead of "<Ctrl>-". If the parse fails, @accelerator_key and @accelerator_mods will be set to 0 (zero).