Title bar button: fixes from trying it in the live desktop

- The decoration's "bottom" property clashed with Item's (KWin refused it).
- No On All Desktops button with one virtual desktop, like Breeze.
- decoration/apply.sh finds the session's D-Bus through plasmashell (KWin's
  environment isn't readable), and installs each try under a new name: KWin
  keeps a decoration's QML by name until it restarts.
- ft-floatd starts its script with Scripting.start: after a reload, the new
  script gets the old one's id while the old one is still being deleted, so
  run() on /Scripting/Script<id> went to the old script and the new one never
  ran (a second ft-floatd start left floating windows without their script).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

decoration/contents/ui/FtButton.qml

@@ -116,5 +116,8 @@ DecorationButton {
             visible = Qt.binding(() => decoration.client.providesContextHelp);
         if (buttonType === DecorationOptions.DecorationButtonApplicationMenu)
             visible = Qt.binding(() => decoration.client.hasApplicationMenu);
+        // Like Breeze: no On All Desktops button with only one virtual desktop.
+        if (buttonType === DecorationOptions.DecorationButtonOnAllDesktops)
+            visible = Qt.binding(() => decorationSettings.onAllDesktopsAvailable);
     }
 }