Consolidate stocks with checking accounts

[slodki]

Idea

• Treat all currencies, crypto-currencies and stock shares as a single entity used to describe transaction/account value.
• Reuse checking accounts for both investment and share accounts and store stock purchase/selling transactions as transfers with AMOUNT and TOAMOUNT values.
• Reuse currency history table for storing stock history too.

Benefits

• Model simplification, model code reuse
• Database simplification - consolidate similar entities, enabler for foreign keys implementation Add foreign key constraints to database scheme #23
• Possibility to reuse generic GUI panels, dialogs etc. Edit Stock Investment dialog errors moneymanagerex#1751
• No need to implement missing GUI elements for shares/investments accounts View Stock Transactions windows needs redesign moneymanagerex#1752
• Eliminate separate implementation for commision and use the same implementation for both commision and payment of the dividend (and fee for transfers beetwen normal accounts)
• Ability to report foreign currency or cypto-currency buying as investments Cryptocurrency portfolio tracker with updates from Coinmarketcap/Livecoinwatch moneymanagerex#1834
• Eliminate current errors in investment/share implementations Stock errors moneymanagerex#1691 Allow transactions with empty payee moneymanagerex#1679
• Ability to define BILLDEPOSITS for shares and use to implement regular contributions to a pension scheme