Big code refactor

The design was defective and I didn't expect the popularity of the project. After these changes the code will be more mantainable and understandable.

Among the changes we can see:
- A better code structure
- Decoupled button widget from its logic
- More code reuse
- Easier way to add buttons
- Specialized classes

src/config/configwindow.h

@@ -28,7 +28,7 @@ class QListWidget;
 class ConfigWindow : public QWidget {
     Q_OBJECT
 public:
-    explicit ConfigWindow(QWidget *parent = 0);
+    explicit ConfigWindow(QWidget *parent = nullptr);
 
 protected:
     void keyPressEvent(QKeyEvent *);