QmlJSInspector: Set property values

QML property values could be set in the inspector.
Regression introduced by 7f09d0b7.
This patch is a partial fix. Only values can be assigned currently.
For expressions, use the console.

Change-Id: I945b11109cd7788dd0f5277af206bf9658844aee
Reviewed-by: Christiaan Janssen <christiaan.janssen@nokia.com>
Reviewed-by: hjk <qthjk@ovi.com>