centralize and fixup example sources install targets

This follows suit with aeb036e in qtbase.

Change-Id: Ie8580d0a1f38ab9858b0e44c9f99bdc552a1752a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: hjk <qthjk@ovi.com>

examples/multimedia/audiodecoder/audiodecoder.pro

@@ -0,0 +1,17 @@
+TEMPLATE = app
+TARGET = audiodecoder
+
+CONFIG += qt warn_on
+
+HEADERS = \
+    audiodecoder.h \
+    wavefilewriter.h
+SOURCES = main.cpp \
+    audiodecoder.cpp \
+    wavefilewriter.cpp
+
+QT += multimedia
+CONFIG += console
+
+target.path = $$[QT_INSTALL_EXAMPLES]/multimedia/audiodecoder
+INSTALLS += target