Treat key combinations as a unit (combos)

The display now only updates on key-down (the combo grows as you press)
and when the last key is released (the empty payload starts the linger
with the last full combo). Intermediate releases no longer shrink the
display, so the order you let go of a shortcut doesn't matter: Ctrl+Shift+N
stays Ctrl+Shift+N whether Ctrl, Shift, or N goes up first, and the 1s
linger shows the full combo instead of a partial release state. Matches
the keybinding-tutorial use case better than keyviz, which shrinks
key-by-key on spaced releases.

manifest.json

@@ -2,7 +2,7 @@
   "schemaVersion": 1,
   "id": "felixzsh.key-visualizer",
   "name": "Key Visualizer",
-  "version": "1.3.3",
+  "version": "1.4.0",
   "description": "Shows the keys you press on screen. Great for keybinding tutorials, demos, and screencasts.",
   "kinds": [
     "panel",