secondary-window: add ability to retract a view

[vince-fugnitto]

Feature Description:

A known limitation of https://github.com/eclipse-theia/theia/pull/11048 is the fact that webviews (for the moment) can only be extracted to a secondary window, but there is no functionality as of yet to retract the view so it returns to the main window. A workaround includes closing the secondary window, and re-opening the view from the main area.