ui: reworked StartScreen and ProjectSettupScreen UI flow

* realigned buttons and elements
* fixed retina styles for mac
* removed MessageLoadSource and subsequent functionality
* edit existing project through menu
* browsing paths from the line item in QtDirectoryListBox
* grow QtDirectoryListBox instead of scroll
* take any file or directory as valid path
* fixed new project setup screen background
* fixed screen handling on cancelation

src/lib/settings/ProjectSettings.h

@@ -10,6 +10,9 @@ class ProjectSettings
 	: public Settings
 {
 public:
+	static std::vector<std::string> getDefaultHeaderExtensions();
+	static std::vector<std::string> getDefaultSourceExtensions();
+
 	static std::shared_ptr<ProjectSettings> getInstance();
 	~ProjectSettings();