Limmit the maximum size of a regular Watch node

##### Background
This pull request places limitation on Watch node's width and height (to design specified values of ```280px``` and ```310px``` respectively). The fix is meant for the following defect:

[MAGN-2446 Watch node gets unusably long](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-2446)

**Note**: This pull request doesn't completely solve everything described in the proposed design, but it makes the Watch node usable again by not allowing it to go crazily long. I will be requesting to lower the priority of this defect and defer it to a later time to address the rest of enhancement works (e.g. display more details of the watched content).

![maximum-watch-node-fix](https://cloud.githubusercontent.com/assets/5086849/2825275/c78283c2-cf47-11e3-9fb5-080a99937725.png)

##### Unit Tests
This is a UI-only change we can't quite make a test case for.