keyboard: "Migrate" gtk-im-context setting before giving control to user

This setting used to be modified by gsd-keyboard at runtime, but it no
longer does. We want to leave this setting in a pristine state before
we lend control to the user in order to avoid setting leftovers make
GTK and others use the unintended IM module.

Since the setting is actually staying on the same schema/path, there is
no nice mechanism that would help us in doing a one-time port, so rely
on a file at ~/.cache to make this happen once. In the common case, it
just adds one stat() more at startup.

After this migration is done, the gtk-im-module setting can be considered
in full control of the user.