value_from_string
Description:
Demarshals a value from a string.
This function calls Value on the value argument, so it 
      need not be initialised beforehand.
Can handle char, uchar, boolean, int, uint, long, ulong, enum, flags, float, double, string, `GdkRGBA` and `GtkAdjustment` type values.
Upon errors false will be returned and throws will be assigned a `GError` from the gtk_builder_error domain.
Parameters:
| this | a `GtkBuilder` | 
| pspec | the `GParamSpec` for the property | 
| value | the `GValue` to store the result in | 
| string | the string representation of the value | 
Returns:
| true on success |