Add support for "none" QT_IM_MODULE

Do not try to load any input method when QT_IM_MODULE is set to "none".

Change-Id: I695cb76d616bb2ce5021979bae2790b2f286122d
Reviewed-by: Pekka Vuorela <pekka.ta.vuorela@nokia.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>