Add single_window_aspect_ratio for master layout

[mike-lloyd03]

Some users on ultrawide monitors do not want a single window to take up the whole screen (#2092, #4414, #5312). Certain fixes have been proposed like adding gaps when a workspace has only one window:

workspace = w[t1], gapsout:10 520

Other solutions like center-master have been proposed as well:

workspace = w[t1], layoutopt:orientation:center
...
master {
    slave_count_for_center_master = 0
}

While these work, there are a few drawbacks:

• The gaps method prevents fullscreen 1 from taking the whole screen since gaps are still respected
• The center-master method doesn't allow defining the width of the single, centered window since it will follow mfact. I might want mfact = 0.5 when tiling multiple windows an mfact = 0.75 when showing a single centered page.

While I still think a max_window_width option would be the best solution, the single_window_aspect_ratio functionality on master would be a suitable solution.

Is it possible to include this functionality on the master layout?