[PY] Started reworking ops addition dialog.

Initial UI support for adding multiple entries at once.
[PY] Moved EntriesDelegate to a separate file.
[PY] Added References::operator=(const PKey<T>&)
[PY] Fixed style in EntriesModel.
[PY] Zero-init entries' Amount_ and Count_ in def ctor.
[PY] Little UI improvements in op dialog.
[PY] Added custom currency rate support for entries.

Breaks storage, upgrade using the following commands:
ALTER TABLE NakedExpenseEntry ADD COLUMN EntryCurrency_ TEXT;
ALTER TABLE NakedExpenseEntry ADD COLUMN Rate_ REAL;
UPDATE NakedExpenseEntry SET Rate_ = 1;
UPDATE NakedExpenseEntry SET EntryCurrency_ = (SELECT Currency_ FROM Account WHERE Account.ID_ = NakedExpenseEntry.AccountID_);
[PY] Support for currencies/rates/prices in EntriesModel.
[Blasq Vangog] Fixed CMake var name clash.