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

docs/dev/qsettings.md

@@ -22,7 +22,7 @@
     - value: "showHelp"
     - type: bool
     - description: show Help messages in capture mode.
-- show Help message
+- show desktop notifications
     - value: "showDesktopNotification"
     - type: bool
     - description: show every desktop notification.