Improve example application

[greyli]

• Fix import statements
• Make SQLite URI prefix compatible with UNIX

This PR also set SQLALCHEMY_TRACK_MODIFICATIONS to False to suppress the warning.

[FesonX]

#17
@greyli set SQLALCHEMY_TRACK_MODIFICATIONS to False MAY effect users.

This MAY cause unexpected behavior if they want to track modification in their program using SQLAlchemy.

I think user SHOULD decide by themselves.

It currently defaults to True in Flask-SQLAlchemy v2.x, and will become False in v3.x.
Ref: How do I know if I can disable SQLALCHEMY_TRACK_MODIFICATIONS?

[greyli]

set SQLALCHEMY_TRACK_MODIFICATIONS to False MAY effect users.
This MAY cause unexpected behavior if they want to track modification in their program using SQLAlchemy.

@FesonX The current default value is None instead of True (detail). I don't think set it to False in the example application will affect users. It will disable the tracking behavior like the default value (None) and also suppress the warning. If the user wants to enable it, they will definitely change this value to True.

[greyli]

@aaajiulong I don't know if I miss something, the src folder is not a package, you can't import like from src.flask_state import .... Normally users need to install the dependencies before running the example application, either from PyPI (pip install flask-state) or from the source (pip install -e .., run inside the examples/). It will be better to add a requirements.txt or README.md for examples.