renderer: attaching `CommentNode`s to visual tree causes problems

[sis0k0]

When ng-template is used, the Angular renderer creates comment nodes. In the context of NativeScript, these comments are invisible elements - Placeholders, which are attached to the visual tree.

Consider the following layout:

<Label text="first"></Label>

<ng-template [ngIf]="false">
    <Label text="second"></Label>
</ng-template>

Since the Page can have only child, the above will be rendered as:

(ProxyViewContainer
    (CommentNode) // anchor for the second label
    (ActionBar)
)

The first label gets overridden by the CommentNode, created for the second label. In the above case, there's an easy workaround - one can simply wrap the labels in a StackLayout. Still, there are more complicated scenarios with no easy solution.