Open an input method. The minput_open_im() function opens an input method whose language and name match
language and name, and returns a pointer to the input method object newly allocated.
This function at first decides a driver for the input method as described below.
If language is not Mnil, the driver pointed by the variable minput_driver is used.
If language is Mnil and name has the property Minput_driver, the driver pointed to by the property value
is used to open the input method. If name has no such a property, NULL is returned.
Then, the member MInputDriver::open_im() of the driver is called.
arg is set in the member arg of the structure MInputMethod so that the driver can refer to it.