Added support for closable tabs.

Tabs can be 'closed' by clicking the 'close' button. TabControl has a CloseTabCommand property where the actual closing of the tab can be handled.

Also, the close button can be enabled on a tab-by-tab basis so it goes hand and hand with #451.

![mmclose](https://f.cloud.github.com/assets/251501/532370/48de178c-c144-11e2-9432-7543960f5970.png)