sl0ppty: watch what the config was built from, not just what it was called

An included theme that does not reload when you save it is half a
feature, and the half that is missing is the one the file headers in
contrib/themes have been promising all along ("a running session re-reads
it the moment you save"). That was true while a theme was something you
pasted; the moment it became something you include, the watch was on the
wrong file.

So a config now remembers every file it was read from -- itself and
everything it included -- and the watcher watches all of them. Recorded
before each parse rather than after, because a file that is not there yet
is exactly the file worth watching: writing it is the next thing that
will happen, and that is the same reason the watch is on directories
rather than on files. Deduplicated, so a cycle does not fill the list
with two names twice.

The set is rebuilt after every successful load, because it is a property
of the config that just loaded: an include can be added, pointed
somewhere else, or taken out, and the watch has to follow. One inotify
watch per directory, deduplicated by the descriptor inotify hands back
for a directory it is already watching; names are matched against the
whole set rather than against the directory whose watch fired, which
means two directories holding a file of the same name reload for each
other's save. That costs a debounced re-read of a config that has not
changed, and is not worth a second data structure to avoid.

Only the main config's directory is created if missing. An include's is
not: a mistyped path should not leave a directory behind.

tests/test_reload.py drives a real session for this one -- the watcher is
the thing under test and the headless driver has none. It writes a config
that includes a theme, checks the theme applied at startup, then edits
*only the included file* and waits for the session to change colour.

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