QmlDesigner.PropertyEditor: Do not allow negative width or height

Task-number: QTCREATORBUG-12025
Change-Id: I5348869e8627ca7de0cd5f3c67bbeb3353770f15
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Revert "MsvcToolChain: correct suggested Mkspec list."

The WinRT xmkspecs detection was broken by this patch.

This reverts commit 8095a212b378d165712766377926434038f0948d.

Change-Id: I6bc1b2dc2e090c6e7bd0cfa1894b678083a1d107
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@digia.com>
Reviewed-by: David Schulz <david.schulz@digia.com>