Create a new clients/ subdirectory at the top level, and move cli/ and
tui/ into it, as well as nm-online.c (which was previously in test/,
which made no sense).
cli/ was split into two subdirectories, src/ and completion/. While
this does simplify things (given that the completion file and the
binary both need to be named "nmcli"), it bloats the source tree, and
we can work around it by just renaming the completion file at install
time. Then we can combine the two directories into one and just have
it all under clients/cli/.
If you launched nmtui directly into the editor for a specific
connection, it would hang with a blank screen when you quit.
Fix this by changing the way startup works a bit, and have the created
toplevel NmtNewtForm get returned all the way to nmtui.c, which can
then connect to the "quit" signal on it and quit (rather than having
the different subprograms trying to guess whether they're supposed to
quit-on-exit or not).
