Handle high contrast themes better when system is in high contrast

When system high contrast is turned on, Storage Explorer colors don't behave very well, even when the app's theme is set to High Contrast -Black or High Contrast - White. More specifically, borders for trees and tables are colored incorrectly.

The reason for this is because when system high contrast is turned on, the OS forces most colors into system colors (defined by the user). This means, no matter how we style certain elements, colors will always be forced to OS configured colors. This happens even if an element is styled with a hex value matching a system color. For example, even if the border color of the tree nodes is set to the correct cyan color, it will still be forced to white.

To better handle this situation, I propose the following:

• When system high contrast is turned on, ignore the app theme setting and use a "special" theme that uses system colors.
• Keep our current high contrast black/white themes as app theme choices for when system high contrast is turned off.