Improved type hints and declarations for \Magento\Quote\Model\Quote\Address\Total

[schmengler]

This PR adds some proper type hints and docblocks related to the quote total.

Description

• For better IDE support, Quote::getTotals() now has a return type of Total[] instead of array (via docblock). I traced down the called methods to verify the type and added missing type hints on the way.

• The TotalFactory has been updated to return Total by default (previously there was no default value, so BC is ensured). The factory is used like a default generated factory in TotalsReader but needed to pass the class name to create(). This violates the POLA. Also,
  the return type hint is adjusted now (Total is no subtype of
  Total\AbstractTotal)

• TotalReader::$baseTotalAmounts and TotalReader::$totalAmounts are now initialized as empty arrays. TotalReader::getAllTotalAmounts() did not return an array if no totals have been added.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)