gui: add active or zoomed merged buffer support to layout (issue #562)

[sim642]

Closes issue #562.

The main change in this PR is the added active field to layout buffers, which additionally gets stored and handled during layout apply. Since the active field for merged buffers is also used to indicate merged buffer zoom to a single buffer, it will also be persisted in the layout for free. Same is true for persisting layout during upgrade.

The secondary change in this PR is fixing gui_buffer_unmerge_all such that the unmerged buffers are in the same order as they were merged, not in the reverse order as before. This is used when applying layouts and is needed to make that idempotent and restore merged buffers in the correct order.

Happy Hacktoberfest!