Proposal: SelectionStart and SelectionLength of TextBox aren't DependencyProperties

# Proposal:
TextBox SelectionStart and SelectionLength should be DependencyProperties

## Summary & Rationale
While they're settable properties to manipulate the selection, since they're not DependencyProperties they can't be TwoWay bound too. So, if you're using MVVM and want to save/restore the selection of your TextBox, you can't for instance without extra code to glue things together.

Start by locating the TextBox implementation and the existing SelectionStart and SelectionLength declarations. Confirm how dependency properties and TwoWay binding are handled for comparable TextBox state, then verify that selection manipulation and binding both work as expected.

Written by the indexing model from the issue text.