Flyout themes

In the end we want to be able to do something like this:

- `FlyoutTheme.Adapt`: Use the theme of the host window
- `FlyoutTheme.Inverse`: Use the inverse theme of the host window
- `FlyoutTheme.Dark`: Always dark. Default value.
- `FlyoutTheme.Light`: Always light.
- `FlyoutTheme.Accent`: Use the host window's accent as theme

Fixes #836

TODO:

- [x] Fix the window commands for the light theme

EDIT: Alright, feel free to test this

Some screenshots:

## Accented
![flyout-accented](https://f.cloud.github.com/assets/964691/1929672/b63fda3a-7e9b-11e3-9df9-2c545faa40f9.PNG)

## Inverse
![flyout-inverse](https://f.cloud.github.com/assets/964691/1929674/bd2ab414-7e9b-11e3-8354-98fb2b352ecc.PNG)
![flyout-inverse2](https://f.cloud.github.com/assets/964691/1929686/f5ce8e1c-7e9b-11e3-9760-114146ced1f7.PNG)

## Adapt
![flyout-adapt](https://f.cloud.github.com/assets/964691/1929769/8d2d7c4a-7e9d-11e3-9b18-d554f1f980fd.PNG)