Allow panel widgets to be null, fix ugly MSBuild build engine UI when MSBuild is required

This fixes two issues:

* First, `OptionsDialog` would NRE when `CreatePanelWidget` returned `null`
* Because of this, CombineMSBuildOptions would return an empty yet instantiated `VBox` if a project *required* the use of MSBuild (and thus the option to enable/disable using MSBuild for building should not be shown). For projects that require MSBuild, this would be ugly as two horizontal lines would be shown with no content in between them:
![screen shot 2014-11-21 at 10 32 55 am](https://cloud.githubusercontent.com/assets/49539/5144847/cb8de648-716b-11e4-91ac-31cfd197827a.png)
This PR fixes the NRE and makes `CombineMSBuildOptions` simply return null for its `CreatePanelWidget` if MSBuild is required for a project:
![screen shot 2014-11-21 at 10 39 24 am](https://cloud.githubusercontent.com/assets/49539/5144868/eeac6ea6-716b-11e4-891f-db8563795298.png)