feat: Selectable backend.

[davidhozic]

Add an option to select the backend.
The backend is an underlaying Tkinter-like library.

Currently supported backends:

• Tkinter: Default backend.
• ttkbootstrap: Automatically enabled if ttkbootstrap is imported before TkClassWizard.

Pending supported backends:

• Custom Tkinter

Required refactoring:

• Change inheritance to composition to allow dynamic change of backend.