Other: change how pkg-config includes are handeles

Wrote some own functions to hande pkg-config includes.
now Makefile generation will stop if xmms2-client-cpp.pc is missing
Now, compilation should work, even if xmms2-client-cpp development files
where installed after qmake das called for the first time.
also enabled adding files in the playlistview contextmenu

lib/lib.pro

@@ -1,6 +1,7 @@
 TEMPLATE = lib
 CONFIG += static
 include (../config.pri)
+load (../features/pkgconfig.prf)
 
 MOC_DIR     = .moc
 OBJECTS_DIR = .obj
@@ -39,7 +40,12 @@ QMAKE_CXXFLAGS += -g
 ;CONFIG += debug warn_on
 QMAKE_CXXFLAGS_WARN_ON += -Wno-unused-parameter
 
-CONFIG += link_pkgconfig
-PKGCONFIG += xmms2-client-cpp xmms2-client
+XMMSCLIENTPKG += xmms2-client-cpp
+pkgconfig_exists ($$XMMSCLIENTPKG) {
+	QMAKE_CXXFLAGS  += $$pkgconfig_cflags($$XMMSCLIENTPKG)
+	LIBS += $$pkgconfig_libs($$XMMSCLIENTPKG)
+} else {
+	error (Please install the xmms2 c++ client library)
+}
 ;CONFIG += avahi