qml: try to show tool tip after its position is adjusted

I previously noted in !6240 (comment 462295):

I have tested this now on Windows 7 and Windows 10. Both seems fine to me. In the future, we can consider delaying visibility so that we ensure that the position is set first.

I did not reproduce this on virtual machine Windows 7 and Windows 10 (nor on X11), but unfortunately it seems to be really noticeable on normal Windows. We can try to position the tool tip before it is shown to mitigate that.

Request review @chub.