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 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_mods will be set to 0 (zero).
string representing an accelerator
return location for accelerator keyval, or null
return location for accelerator modifier mask, null