[birt-dev] Checkin: fix bug 160629:Lost the ability of thumbnail in repo

["Xiong Zhang" <xiong.zhang@xxxxxxxxxxx>]

-Summary:
Lost the ability of thumbnail in report design
 
- Bugzilla Bug (s) Resolved:
  160629
 
- Description:
  we can't clear the preivew image once we set it.If the report has a
preview image, then the thumbnail can't be shown.
 
- Tests Description: 
Manual
 
- Notes to Build Team:
 
- Notes to Developers:
 
- Notes to QA:
 
- Notes to Documentation:
 
- Files Added:
 
- Files Edited:
/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designe
r/internal/ui/views/attributes/widget/PropertyDescriptorFactory.java
/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designe
r/internal/ui/wizards/WizardReportSettingPage.java

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