In case ProgressMonitorFocusJobDialog is created but never shown, we
leak lot of connected SWT resources. This can happen if a modal dialog
was opened already before the dialog was requested.

Additionally fix a potential case where opened dialog couldn't be closed
because the cancelled job.

Change-Id: Ia2c217295e911a7e2dcf799d73e6f5627e112b24
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>