Make Qt Data Visualization examples self-contained

The examples included ../examples.pri, which injected the in-tree
include and library paths (INCLUDEPATH += ../../../include and
LIBS += -L.../../lib). These build-tree paths reference files outside
the example directory, violating QUIP-13 and breaking the "copy example
out of tree" feature of Qt Creator and VS Code.

Each example now inlines only the settings needed for a self-contained,
out-of-tree build against the installed module (TEMPLATE = app, the
required QT modules, and the install target). The in-tree include and
library hacks are dropped, and examples.pri is removed.

Task-number: QTBUG-147520
Pick-to: 6.11 6.12
Change-Id: Ifa6c6f9e42319753a0073b8a39ce05617b31221c
Reviewed-by: Kai Köhne <kai.koehne@qt.io>