Fix bug where cut and paste would not correctly work on SplitButtonPage and RichEditBoxPage

[marcelwgn]

Description

Fixed bug #286 by detecting when user has changed text upon REB gaining focus, to not interfere with that.

Motivation and Context

Fixes #286

How Has This Been Tested?

Tested by reproducing steps in #286:

1. Launch XAML Control Gallery app.
2. Navigate to "SplitButton" or "RichEditBox" in All controls and activate it.
3. In the textbox (or last sample of RichEditBox page), select the text written and right click to invoke the context menu and select "Cut" menu item.
4. Again invoke the context menu and activate "Paste" menu item and observe.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)