clients: reorganize source tree, put all the installed clients together

clients: reorganize source tree, put all the installed clients together

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/.