Bugfix: ignore _NET_ACTIVE_WINDOW for scratchpad windows (Thanks mistnim)

fixes: #1136
Merge branch 'fix-active'
Merge branch 'master' into next
add release notes for v4.7
Merge branch 'release-4.7' into next
debian/: add placeholder commit
use splith as default when restoring a layout

This frees users from specifying layouts even when they don’t make any
difference, e.g. leaf nodes.
swallows: value needs to be loaded as regular expression
tests: add simple layout restore test
layout loading: allow comments in JSON files
layout restore: support more criteria, match only once (+test)
GET_TREE: serialize container type into a string

So far, this was blessed for internal use only (by virtue of not being
in the documentation), but we want to expose it for the stored layouts.
allow parsing multiple nodes from a json layout
load_layout.c: use *LOG instead of (f)printf
add debug message to trace match deletions
layout restore: create and render placeholder windows

This is not entirely done yet, but TODO entries are placed in the code
where appropriate.
display swallows criteria in placeholder windows
use root window color depth instead of hard-coded 24
set original window title on placeholder windows
introduce client.placeholder color