sway/window: app_id on unfocused workspaces

[kris7t]

This patch tries to make app_id specific styling from sway/window more useful if the smart_borders and smart_gaps Sway options are used in conjunction with the offscreen-css option of the module.

My main use case for this is styles like window#waybar.solo.kitty, which can make Waybar blend in with the solo window on the workspace (gapless and borderless thanks to the Sway options). For example, you may set a semi-transparent background for Waybar only if a terminal with semi-transparent background is the only window on the workspace, but use a fully opaque background for any other (opaque) window.

Unfortunately, app_id-specific styling is not very useful with multiple monitors. Even if offscreen-css is enabled, the CSS class corresponding to app_id is only applied for the focused window. Thus, Waybar can only ever blends with focused windows.

This patch also applies the app_id for unfocused workspaces when offscreen-css is set, but only if there is a single, non-floating window. This should catch situations where smart_borders and smart_gaps are useful. Technically, gaps and borders are collapsed also when the workspace itself is set to tabbed/stacking with multiple windows, but in that case titlebars (for tabs/stacks) are always present, which would prevent the bar from seamlessly blending in with the visible window anyways.

Since this changes established behavior and might break some configs, a more prudent alternative approach would be to add another configuration option to be used in conjunction with offscreen-css to enable this behavior.

[Alexays]

Thanks!