some refactoring

* fix undefinded behaviour
* drop dbus timout, use generic timeout insead
* drop load source to own cmake key
* update contributing.md

sources/test/testdesktopsource.cpp

@@ -39,11 +39,6 @@ void TestDesktopSource::cleanupTestCase()
 void TestDesktopSource::test_sources()
 {
     QCOMPARE(source->sources().count(), 4);
-    // FIXME there is segfault here sometimes o_0
-    //    QVERIFY(std::all_of(
-    //        source->sources().cbegin(), source->sources().cend(),
-    //        [](const QString &src) { return
-    //        src.startsWith(QString("desktop/")); }));
 }