Allow negative values (on CSV import, and in UI)

[aanno]

This PR is a WIP/Draft for relaxing the restriction of providing positive values for the most input and CSV fields.

Here are some relevant links for discussion why this might be useful:

• Allow Negative Tax and Fee (Credits) #3450
• Possible improvements to (transaction) CSV importer #3616
• CSVAccountTransactionExtractor: Do not blindly ignore amount sign #3617
• https://forum.portfolio-performance.info/t/verbuchung-von-anleihen/1537/81
• Allow dividends to be negative
• Eintragen von negativen Dividenden
• Import von negativen Dividenden
• Negative Dividenden bei Shortpositionen

What works now:

• Importing CSV with negative values now works
• number of shares, fees, taxes and the (net) amount could be negative now (CSV, UI)
• 0 (zero) for gross (amount) now works

What does not work (needs improvements):

• Negative value for gross (amount) and dividendAmount does not work so far
• I have broken the tests, especially in name.abuchen.portfolio.tests and name.abuchen.portfolio.ui.tests

This is a draft implementation not ready for merging. It's main purpose is to find out if such a contribution would be accepted. The real implementation should

• fix the missing parts (see above)
• fix the tests (see above)
• make this feature an experimental function that could be switched on in the settings

[aanno]

What works now:

• importing CSV with negative values
• number of shares, fees, taxes and the (net) amount could be negative (CSV, UI)
• negative value for gross (amount) and dividendAmount
• 0 (zero) for gross (amount)
• all tests

Closed in favour of #5445