fix some metro circle button style issues

- add a FocusVisualStyle
- fix background (only ellipse should get the background)
- capture mouse only if it's over ellipse

![image](https://f.cloud.github.com/assets/658431/1191955/a769537e-245a-11e3-9685-1e2f83ccf378.png)

if no one finds a bug or has some suggestions, i merge this as soon as possible.