IBus input method panel's initial position is wrong when switching from other app to qt app

When using IBus input method, if you first typing in other apps, and then focus a Qt app, start typing something, the input candidate panel is still shown in the previous app (previous position), only after close and re-show the panel it will shown in Qt app.

This is because we don't call `cursorRectChanged` after `focusIn`, so the initial position is not updated.

I've already created a patch for this: <https://codereview.qt-project.org/c/qt/qtbase/+/561525/1>.