Set default layout per monitor

I would like to be able to configure which layout gets used on each specific monitor when I either log on or add the monitor to the system while it is running.

Example:
I have three profiles:
90/10 Horizontal
60/40 Horizontal
70/30 Vertical

When i log on, my main monitor is 90/10 Horizontal, though I mainly use it in 60/40.
When i occasionally switch my secondary vertical monitor on using my HDMI switcher, it comes in as 90/10, though i would like it to be 70/30 vertical.

I would like to be able to define independent defaults for my main and secondary monitors.

Start by tracing how layouts are selected when the session starts and when a monitor is added or reactivated. Identify the existing profile and monitor-detection entry points, then define how independent defaults for the main and secondary monitors should be stored and selected; done means each monitor receives its configured layout in both scenarios.

Written by the indexing model from the issue text.