Prevent flicker on MiniProjectTargetSelector show

Reproducible on Mac OS X.

Set position before changing the visible state.

Change-Id: I8a7e73dae94ceb865ea8a7ef4cec4112220888de
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>