Tries to parse the given string into a trigger.
On success, the parsed trigger is returned. When parsing failed, null is returned.
The accepted strings are:
- `never`, for `GtkNeverTrigger`
- a string parsed by accelerator_parse, for a `GtkKeyvalTrigger`, e.g. `< Control>C`
- underscore, followed by a single character, for `GtkMnemonicTrigger`, e.g. `_l`
- two valid trigger strings, separated by a `|` character, for a `GtkAlternativeTrigger`: `<Control>q|<Control>w`
Note that you will have to escape the `<` and `>` characters when specifying triggers in XML files, such as GtkBuilder ui files. Use ` <` instead of `<` and `>` instead of `>`.
the string to parse
a new `GtkShortcutTrigger` or null on error