rhbz#1079672: FileDialogHelper: don't hand out stale preview Graphic

The maGraphic is updated from a timer, and if you double-click on a file
then the dialog may exit with the previous file still previewed in
maGraphic, so the wrong image is returned by the dialog.

Change-Id: I99094d85d8d68d5c8a842f52e7039dbbbf095995
remove unnecessary references to test-install (just use instdir)

Change-Id: I99f19a3e2ed166c2ea4397f8767975973dd5d983
Consistency around SdrMetricItem in svx/sxmlhitm.hxx

...similar to what has been done for svx/sdtmfitm.hxx in
6a2ea81ca1622d2c2ad55bea8ddc28167fcc2794 "Remove unused ctors" and
68969cc61adecac481ae9656978ef952f435b310 "Consistency around SdrMetricItem."

Change-Id: I65c36bde166c5f78db8c76364fd057017982fed6