[SPARK-17677][SQL] Break WindowExec.scala into multiple files

## What changes were proposed in this pull request?
As of Spark 2.0, all the window function execution code are in WindowExec.scala. This file is pretty large (over 1k loc) and has a lot of different abstractions in them. This patch creates a new package sql.execution.window, moves WindowExec.scala in it, and breaks WindowExec.scala into multiple, more maintainable pieces:

- AggregateProcessor.scala
- BoundOrdering.scala
- RowBuffer.scala
- WindowExec
- WindowFunctionFrame.scala

## How was this patch tested?
This patch mostly moves code around, and should not change any existing test coverage.

Author: Reynold Xin <rxin@databricks.com>

Closes #15252 from rxin/SPARK-17677.