Have support for querying preedit rectangle for input methods

Some input methods could use the ability for getting preedit rectangle from widgets in order to avoid it when showing graphical elements of their own on top of the widget. QWidget::inputMethodQuery() supports Qt::ImMicroFocus query for getting the cursor position. This is not enough, so meegotouch has extension query value M::PreeditRectangleQuery.

Either a similar query value should be implemented in Qt or the description of ImMicroFocus should be changed to mean preedit rectangle when preedit exists. However, preedit can also include a cursor so just using a separate query would be clearer. And on meegotouch we also need the cursor rectangle inside the preedit for our virtual keyboard. [*]

• Side note: Qt widgets don't yet properly support this case, and the documentation is not clear if the return value should be actual cursor position, i.e. where preedit starts or ends, or what was passed to widget with QInputMethodEvent's cursor attribute. I would vote for whatever cursor being visible.