Add possibility to dock/convert to tab the main window

We should be able to dock the main window as a tab.

This is not possible with the current API, it will require some changes in the core + some changes in the logic of the plugin or a new way to get the active sprite.

Notes/impl problems: the "active sprite" is deducted by the active tab, if we convert the attachment system in a tab, the active tab could be the attachment system or the sprite editor so what would be the "active sprite" when the attachment system is selected? the latest active sprite?

Start by reviewing the current core API and the attachment system's active-sprite logic. Resolve how the active sprite should be selected when the attachment system or sprite editor is the active tab, then define the changes needed to dock or convert the main window to a tab. Done means the window can be tabbed without ambiguous sprite selection.

Written by the indexing model from the issue text.