feat(portfolio): basic support for multi-currencies

[MadeInPierre]

Description

Let the user set its own currency symbol. Several options:

• Line(..., currency="$") to set a single line's currency
• Folder(..., currency="$") to set every children's currency at once (used as a shortcut)
• Portfolio(..., currency="$") to set the default global currency everywhere

Changes made in a child take priority over a change made in a parent.

Actions

• Closes #54

Steps

• [X] Set the currency symbol for an individual node
• [X] Set the currency of every children in a Folder
• [ ] Behavior when a Folder has multiple currencies? ➡ For now, simply write a '?' symbol
• [x] Create a global Finalynx config with a default currency symbol? ➡ no need for now, simply set the Portfolio's currency