Fix coding style

[ryonakano]

Changes Summary

Overall, I tried to follow the elementary coding style.

• Explicit private
• Always use brackets even if there is only one line in an if statement
• Newline after closing brackets except when followed by another closing bracket or an else statement
• Don't declare an instance of MainWindow as Gtk.Window
• Space before a paren
• Use construct sentence instead of the one-time-used build_ui method
• Reversed condition to avoid nesting if statement
• Fix long lines (over 120 words)
• Signals at the beginning of the class
• Use 4 spaces instead of tabs for indentations
• Spaces in properties
• Lesser scope when possible (private set)
• Remove an extra space before a bracket
• Remove unused methods
• Use GObject construction

If you feel this PR contains too many changes at the same time, please let me know so that I'll separate it into several ones.

[Djaler]

Thanks!