New budgeting system

(from [7]) [#2 New budgeting system] added budget transaction/entry. however, the code is highly ugly for now (there's a separate budgeting system for transactions and for sheets/graphs).

(from [8]) [#2 New budgeting system] Refactoring. Created model.budget and moved some of the logic from model.oven over there.

(from [10]) [#2 New budgeting system] The budget spawns are now placed at the end of every recurrence period.

(from [11]) [#2 New budgeting system] Converted the Account.budgeted_amount() logic to the new recurrence based logic. (whew, this was complicated! and it ain't even completely done yet...)

(from [12]) [#2 New budgeting system] Moved the budget calculations up in Document (in preparation for them being schedule based rather than account based)

(from [43]) [#2 New budgeting system] The BudgetSpawns now have budget targets among their splits.

(from [44]) [#2 New budgeting system] Budget now increment balances in etable again. This required a new balance field in Entry, and some little refactoring.

(from [45]) [#2 New budgeting system] Started the process of moving budgeting off the Account class. For now, changing budgets also change an additional list of Budget instances, which will replace Account.budget and Account.budget_target soon enough.

(from [46]) [#2 New budgeting system] Removed budget info from Account. It's now exclusively in BudgetList.

After re-thinking about it for a while, I came to the conclusion that this "Budget" checkbox in the transaction panel was not a very intuitive way of setting budgets and that I should implement #3 View for scheduled/budgeted transactions before going any further with this ticket.

(from [76]) [#2 New budgeting system] 2 budgets with the same account will now correctly be computed.

(from [77]) [#2 New budgeting system] Fixed calculations for budget with a repeat type other than 'Monthly'.