doc: QtPlayer: update .pro file to use pkg-config

It allows to configure the example with, for instance, the following
command line:

    PKG_CONFIG_PATH=/usr/local/lib/pkgconfig/ qmake ..

PKG_CONFIG_PATH can be adapted to the path where libvlc has been
installed and/or where Qt is available.

This also remove the unused parts, and add QT += widgets which is needed
to compile.
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merge request