logic: cleaned up code for SourceGroupSettings and Wizzard

* Flattened sourcegroupsettings class hierarchy by moving cross compilation options to own base class
* QtProjectWizzardContentFlags constructor required correct settings object to reduce the need for type casting
* QtProjectWizzardContentPath constructors required correct settings objects
* moved SourcePaths from SourceGroupSettingsJava to SourceGroupSettingsJavaEmpty because they are not required for Gradle and Maven projects
* removed classpath from maven and gradle source group settings
* reuse code of SourceGroup for "show source files" in wizard

src/lib/settings/SourceGroupSettingsJavaMaven.cpp

@@ -85,3 +85,8 @@ void SourceGroupSettingsJavaMaven::setShouldIndexMavenTests(bool value)
 {
 	m_shouldIndexMavenTests = value;
 }
+
+const ProjectSettings* SourceGroupSettingsJavaMaven::getProjectSettings() const
+{
+	return m_projectSettings;
+}