Issue on pin/unpin when pane's headerId is defined

[teodosiah]

Description

When headerId of some pinned pane is set to slot like for example the following:

 <div slot="header1" class="header-slot">
    <span >Pane 1</span>
    <div class="additional-buttons">
    <button #btn igxButton="icon" (click)="deleteButton(btn)">
        <igx-icon class="custom-button">delete</igx-icon>
    </button>
    </div>
</div>

and the user unpin the pane it's header is not visible in the area where all unpinned panes are displayed.

Steps to reproduce

1. Create IgcDockManager with several panes
2. Set the slot above to the headerId of some of the panes
3. Unpin this pane

Result

Expected result

[DiyanDimitrov]

Since a slot can be projected at a single place in the DOM, we have decided to expose different slot properties for each location the header is displayed:

• headerId - slot rendered in the content pane header
• tabHeaderId - slot rendered in the tab header
• unpinnedHeaderId - slot rendered in the unpinned pane header

If any of these slot properties is not set, the value of the header property will be rendered.