Make a disabled Textbox, RichTextbox and PasswordBox a bit more visible

Fixes #1410 

**Before**
![2014-06-13 16 22 30 mahapps metro - demo application](https://cloud.githubusercontent.com/assets/964691/3270976/3f221b04-f306-11e3-891c-8e74277fbb0c.png)

**After**
![2014-06-13 16 21 39 mahapps metro - demo application](https://cloud.githubusercontent.com/assets/964691/3270982/47e2bac8-f306-11e3-873d-dcbb26f2a2dc.png)


**The difference is more noticeable when you fire up the MetroDemo for yourself**