Qt: fix multiple instances of a singleton if an input is already started

Using THEMIM at this level results in instantiating multiples copies of
what should be a singleton. This is because any preexisting input needs
to be initialized in the constructor of the THEMIN (MainInputManager). This
leads to recursively calling the constructor over and over.

This patch directly calls the underlying functions(no functional change).
Qt: fix missing slot (playlist entry of popupmenu)
Qt: early instantiation for THEMIM

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).