don't install mac bundles atomically

... as that causes debug+release installs to overwrite each other's
postprocessed files.

introduces CONFIG+=sliced_bundle, which instructs qmake to create
file-by-file install commands. we don't know whether people are not
putting files outside qmake's knowledge into the bundle build dir, so
this mode is not necessarily backwards-compatible, and thus off by
default.

Task-number: QTBUG-28336
Change-Id: I23e90985ccd3311f0237ed61aadca6d7ed8325b7
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>