Do not run event loop in installer.execute()

The event loop somehow breaks the script interface, e.g. attached
UI files cannot be accessed any more ...

This might let the UI block though.

Task-number: QTIFW-464
Change-Id: Ia53cb355be75241ba343cf3abc6fe42bec5be696
Reviewed-by: Niels Weber <niels.weber@digia.com>