Manage multiple portfolios

[ilcardella]

Add the functionality to open a new trading_log.json and instead of update the window with the new data, a new tab should open containing information of the newly opened portfolio.
Ideally the main window should be able to contain a tab for each portfolio.

The UI should already be able to support multiple tabs.
The Backend instead will need a small refactor, probably just allow to handle multiple instance of the Portfolio module