Stop telling every desktop that it is KDE

Only GNOME was recognised, and everything else was handed to KWin. On i3,
XFCE, Cinnamon, MATE, sway and the rest, Dikte wrote an entry into
kglobalshortcutsrc that nothing reads, called the session KDE, and
promised that the keys would work after the next login. They never did.

There is no backend to write for any of them. The /dev/input listener is
already desktop-agnostic, so those sessions are the case macOS has always
been: no registry, nothing to install, nothing to remove, and the
combination held by the running process. One backend() function decides
which of the four this session has, and the name shown, the status read
back, what Install writes, what Settings explains and what the installer
promises are all taken from it, so they cannot disagree.

A desktop now only counts when the program that writes its registry is
there too. A GNOME session without gsettings and a Plasma one without
kwriteconfig6 fall to the listener rather than to a file, which is also
how Plasma 5 stops erroring on a kwriteconfig6 it never had.

What Settings shows on those desktops is the truth: no Install button, no
KWin, no listener checkbox (it is the mechanism, not a choice), what
reading /dev/input costs, that the focused application sees the keys too,
and the command to bind if you would rather your desktop owned them. The
evdev listener records what it is listening for the way the Carbon one
does, so the status line has something to say there at all.

Closes #28

dikte.py

@@ -365,8 +365,8 @@ class Dikte:
 
     def _external(self, name, handler):
         # The built-in listener sees the key press the instant it happens, so a
-        # toggle arriving right behind one is the KDE shortcut catching up on
-        # that same press. Its lateness is also the proof we were waiting for
+        # toggle arriving right behind one is the desktop's own shortcut catching
+        # up on that same press. Its lateness is also the proof we were waiting for
         # that the shortcut is live, which leaves the listener with nothing to
         # do but double every press.
         # Where nothing was installed there is no shortcut to catch up, and