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/SourceGroupSettingsJavaEmpty.h

@@ -2,12 +2,24 @@
 #define SOURCE_GROUP_SETTINGS_JAVA_EMPTY_H
 
 #include "settings/SourceGroupSettingsJava.h"
+#include "settings/SourceGroupSettingsWithClasspath.h"
+#include "settings/SourceGroupSettingsWithSourcePaths.h"
 
 class SourceGroupSettingsJavaEmpty
 	: public SourceGroupSettingsJava
+	, public SourceGroupSettingsWithSourcePaths
+	, public SourceGroupSettingsWithClasspath
 {
 public:
 	SourceGroupSettingsJavaEmpty(const std::string& id, const ProjectSettings* projectSettings);
+
+	void load(std::shared_ptr<const ConfigManager> config) override;
+	void save(std::shared_ptr<ConfigManager> config) override;
+
+	bool equals(std::shared_ptr<SourceGroupSettings> other) const override;
+
+private:
+	const ProjectSettings* getProjectSettings() const override;
 };
 
 #endif // SOURCE_GROUP_SETTINGS_JAVA_EMPTY_H