<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.

[tomasherceg]

Let me provide an update to the issue - the CompositeControl started to be a preferred way for implementing components in DotVVM, and it builds its contents in the Load phase.
We should update the PerformMasterPageComposition method to not throw even when the page contains Content controls that do not have corresponding ContentPlaceHolder yet, and it should check for them after the Init and also after the Load calls on the control tree (I believe this is called somewhere in DotvvmPresenter). Only after Load is finished, we should ensure that all Content controls have the corresponding ContentPlaceHolders, and we should ensure this works in nested master pages as well.