Save the Extrato spreadsheet directory in a configuration file

[CarlosOliveira1988]

Is your feature request related to a problem? Please describe.

Since each user usually has just one Extrato spreadsheet, this feature will be helpful to reduce the time to open the wished spreadsheet

Describe the solution you'd like

Use the "python-dotenv" library to manage the .env files, handling the "EXTRATO_PATH" parameter

Describe alternatives you've considered

1. If the ".env" file does not exist, then create one ".env" file with the default empty spreadsheet
2. If the ".env" file exist but the "EXTRATO_PATH" file does not exist, then open the default empty spreadsheet
3. When closing the application by normal operation (using the "X" button, for example), then save the current file in the ".env" file

Additional context

N/A

[CarlosOliveira1988]

Feature created in the "usage-improvements" branch. Please, check the results.