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.

doc/libvlc/QtPlayer/QtVLC.pro

 TEMPLATE = app
 TARGET = qtvlc
-DEPENDPATH += .
-INCLUDEPATH += .
-LIBS += -lvlc -lX11
 
-# Input
+CONFIG += link_pkgconfig force_debug_info
+PKGCONFIG = libvlc
+QT += widgets
+
 HEADERS += player.h
 SOURCES += main.cpp player.cpp