Disallow installing over existing installation.

Task-number: QTIFW-278

Change-Id: I5e435ac95c2efd699c1515592cdaaf698c3bc215
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>
Merge remote-tracking branch 'origin/1.3' into 1.4

Change-Id: Icc3218f4f33aa455f22629d0d9f4eaa3db324ef8