DUI: class button bug(not highlighted)

I assume, that Ben has big screen :) That's why he cannot reproduce this bug.

Start state
SelectedIndex = -1
![image](https://cloud.githubusercontent.com/assets/8158404/4963885/d55c5ffc-6733-11e4-80ce-bda3a8cea0b8.png)

Open looong class details
SelectedIndex = 1
![image](https://cloud.githubusercontent.com/assets/8158404/4963890/6158f4f2-6734-11e4-8f32-d97ef7d98d60.png)
![image](https://cloud.githubusercontent.com/assets/8158404/4963894/ab042dd8-6734-11e4-99b6-7ce424e9b36b.png)

Select class button, that is under number 5 now (i.e. File). But you have to make place under File shorter, so that class detail doesn't have enough space to expand.
SelectedIndex = 4.
As well as class details don't have space, scroll viewer repaints itself. And in this  place occurs Selection 2 times. First time, when SelectedIndex = 4 and second time, when SelectedIndex = 6(class details).
![image](https://cloud.githubusercontent.com/assets/8158404/4964118/29ceaf7c-6752-11e4-9bb1-a8484abbc805.png)

So, what I've done. We can use `isMouseDown` flag to detect second wrong selection. On `PreviewMouseDown` we will set it to true and reset to false after selection. So that second selection will not work.

Reviewers
@Benglin ,@vmoyseenko, please take a look.

P.S. I'd better talk about it with video.
P.S. P.S. and I tried out use selected index for visibility, it doesn't work, because problem is not in visibility... Problem is in selected index itself.

[Old PR](https://github.com/Benglin/Dynamo/pull/68)
```isMouseDown = false``` should be done right after ```isMouseDown``` is done being used. There is another ```return``` above this that will leave ```isMouseDown``` as ```true``` forever.