layout restoring: append at the nearest split container (or workspace) (Thanks chris)

Before this commit, leaf containers (such as terminal emulators) would
get children appended, which is not intended.

fixes #1223
Don’t overwrite existing windows with placeholder windows

This could happen when you appended a layout on a workspace where there
are already other windows.
respect CFLAGS in linking command

fixes #1233
fix parallel make

fixes #1232
build i3-save-tree(1) manpage (Thanks xeen)
Fix i3.mk to not rebuild test.* all the time

This problem was introduced in 5181055a
add docs/layout-saving

Feedback on this new document is very much appreciated, please don’t
hesitate to state anything that is hard to understand/could be improved.