bug#44266: Transient input methods

[Juri Linkov]

>> The following patch addresses such issues of the current input method
>> implementation by providing a new feature of transient input methods.
>
>> A transient input method is enabled temporarily for entering a key sequence,
>> and is disabled automatically afterwards.
>
> I like the idea, I think.  But just thinking out loud without having
> read the code closely -- would it perhaps be even more useful to have
> the transient method in place until you `C-x \' again?
>
> Then you could
>
> `C-x \ ...  type a couple of words in that method C-x \'

This is achievable already by shorter key sequence:
`C-\ ...  type a couple of words in that method C-\'

> I'm not at all sure, but I think that perhaps people would more often
> type in a couple of words than just a single key sequence?

This feature is intended as an equivalent of the X Compose Multi_key
that inserts a single character, whereas X layouts switcher that activates
a layout to insert a couple of words corresponds to Emacs non-transient
input methods switched by C-\.