added sending closing event message to settings windows (#7)

Reviewed-on: #7
Co-authored-by: Aleksandr Neychev <alexnejchev73@gmail.com>

README.md

@@ -81,19 +81,26 @@ the next time the window is asked for.
 the layout tracking. No UI, and it must stay that way — whatever lands there lands
 in the background process.
 
-The connection between the two is `settings.json` and nothing else. The window
-writes it — whole, into a temporary file moved into place in one step — and then
+Everything the window has to say to the agent goes through `settings.json` and
+nothing else. The window writes it — whole, into a temporary file moved into place in one step — and then
 posts a registered window message to the agent, which re-reads. The message carries
 no data: sending the changed values along would make the file stop being the only
 source of truth. An agent that is not running is a normal case — the window works the
 same. Nobody watches the file: it has one writer, and that writer speaks up.
 
+The one word in the other direction is the last one: Exit in the tray menu asks an
+open settings window to close before the agent goes, so that no window is left
+belonging to nothing. It travels as a named event of the session rather than a
+window message — the agent starts that process and deliberately keeps no handle to
+it. Nobody listening is the usual case, and it is a normal answer.
+
 ## The tray
 
 The settings window is a guest on the screen rather than the app itself: it shows
 up after the installation and whenever the icon or its menu is asked for. Both
 buttons in its title bar mean what they say — the window closes and its process
-ends. The way out of the app itself is the Exit item of the tray menu.
+ends. The way out of the app itself is the Exit item of the tray menu — and it
+closes an open settings window along with the agent.
 
 The menu of the icon is a system one, drawn by Windows. Its captions still follow
 the language chosen in the settings, but the theme no longer reaches it: a WPF