Automatically enable completer for layouts that require it

This series allows to configure layouts that require a completion engine via (ibus,<engine>:<lang>) in org.gnome.desktop.input-sources:

$ gsettings get org.gnome.desktop.input-sources sources
('xkb', 'us'), ('xkb', 'se'), ('xkb', 'it'), ('ibus', 'varnam:ml'), ('ibus', 'varnam:ta')]

This allows us to use e.g. hunspell for xkb layouts and varnam for the ml and ta.

I'm marking this as draft as (due the lack of scheme files) i couldn't check the language switching yet.

Note that this currently needs the following gsetting:

gsettings set sm.puri.phosh.osk completion-mode "['manual']"

/cc @arun-mani-j as this could be useful for the ongoing work to improve varnam integration.

Closes: #16 (closed)