<dot:ContentPlaceHolder> should be supported even inside templates

[tomasherceg]

Currently, the ContentPlaceHolder cannot be used in a template because templates are instantiated in the Init phase which is too late.

            <bs:Panel>
                <HeaderTemplate>
                    <h1>{{value: Subtitle}}</h1>
                </HeaderTemplate>
                <ContentTemplate>

                    <dot:ContentPlaceHolder ID="MainOverview">
                    </dot:ContentPlaceHolder>

                </ContentTemplate>
            </bs:Panel>

We should allow the master pages to work even when the ContentPlaceHolder controls are not available yet, and chech that all of them are resolved after the Init event ends.
This would enable to use them in templates.

We should also check out how it affects the SpaContentPlaceHolder.