Handle MinGW case in Add Library Wizard

Task-number: QTCREATORBUG-10423
Task-number: QTCREATORBUG-7641
Task-number: QTCREATORBUG-6093
Task-number: QTCREATORBUG-9012

Change-Id: Ibd709735fd90d635f34e42367b188fce79013fb5
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Check if project and its target are not null

Task-number: QTCREATORBUG-10437

Fix also the code, so that we always behave like target platform, not like
host platform.

Change-Id: I4d8705ca858f9a520dcd6a780e7ba2f2dd6cb047
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
AddLibraryWizard: wrap text on snippets label

In this way dialog doesn't grow in width when the label contains long
lines.

Change-Id: Ibd5906912c18c8e4b27959ff5a7aeda258d6995a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>