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

features/pkgconfig.prf

@@ -0,0 +1,24 @@
+defineTest(pkgconfig_exists) {
+	packages = $$1
+	count(packages, 0): return(false)
+
+	system(pkg-config --exists $$packages) {
+		return(true)
+	} else {
+		return (false)
+	}
+}
+
+defineReplace(pkgconfig_cflags) {
+	packages = $$1
+	count(packages, 0): return(false)
+
+	return ($$system(pkg-config --cflags $$packages))
+}
+
+defineReplace(pkgconfig_libs) {
+	packages = $$1
+	count(packages, 0): return(false)
+
+	return ($$system(pkg-config --libs $$packages))
+}