Fixed: CBN creation issue with double clicks

Background
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Due to another [completely irrelevant change](https://github.com/Benglin/Dynamo/commit/54f7fa6bbe3ec964761dc886bdb442426bb1eb49), a ```Code Block Node``` can no longer be created through double-clicking. The focus was stolen, most probably as a result of calling ```AggregateUpstreamRenderPackages``` method, causing the ```Code Block Node``` to lose focus (and then disappear) right after double-clicking on the canvas.

This pull request also re-enables the preview icon for ```Code Block Node```.

Changes
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The following are the two changes in this pull request:
* ```StateMachine.HandleLeftButtonDown``` method now retains the focus in CBN creation scenario
* ```NodeViewModel.ShouldShowGlyphBar``` returns ```true``` if either one of the following condition is true:
 * ```NodeViewModel.IsPreviewInsetVisible``` property is ```true```
 * ```NodeViewModel.ArgumentLacing``` is not set to ```LacingStrategy.Disabled```