QmlDesigner.NodeInstances: Fixing enums for int properties

There is no easy way to convert an enum (e.g. Qt.Vertical) to the
integer value without the meta object/QMetaEnum.

For this reason we evluate the enum as an expression
if the target property is int.

Without this patch enums do not work at all for properties defined as int.
Change-Id: I9363a84eef1bf4a3ed2c40b35f7439e249e9bd98
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>