Allow marking certain layouts as favourite

[mataha]

This commit allows the user to mark certain layouts as favourite for use with new_window_with_favourite_layout command, which, much like new_window_with_current_layout, opens a new window; the difference is that said marked layout is used as a base instead of the current one.

If there are multiple layouts marked as favourite, only the first (per natural ordering in saved_layouts) is chosen.

Why this PR? Most of the time I want to open a new window with the same layout as always. Yet, all I have is that single one - using new_window_from_saved_layout for that feels counterproductive. Furthermore, I can't use neither it nor new_window_with_current_layout if I'm doing so from e.g. command line when no other windows exist. This is a partial remedy; I'd like to follow it up with a PR introducing a setting that would make Sublime automatically open windows using a certain layout. I haven't thought about the specifics yet - one idea is to use the marker introduced in this PR.