Rather than wait for the first call to THEMIM to instantiate this singleton,
do it explicitly at an early stage of the plugin initialization.

This is useful for Qt as a dialog provider, since otherwise THEMIM doesn't
get a chance to be set up until, e.g. the first call to a popupmenu,
which then makes this menu slightly longer and noticeable to display.

Also, remove the unneeded instantiation in the main_interface. (THEMIM has
already been used a few lines earlier, which makes the call useless).