QbsProjectManager: Issues pane is not cleared before reparsing

Steps to reproduce:

• create a simple Qbs project
• add some errors to the qbs files
• wait for parsing has been done - check Issues pane that errors/warnings are displayed
• choose another build configuration or kit and wait again for reparsing done

Current result:

• errors/warnings are added to the former result

Expected result:

• issues should be displayed only for the last parsing/build