Add SayoDevice support, 15 key pads and theme styles

Second protocol family (8089:000b) behind a new backend seam, 15 key and 3 knob pads, Knobs North plus the style and colour grid, Windows import fix, window resize fix.

macropad_gui/backends/__init__.py

@@ -0,0 +1,34 @@
+"""
+One device family per backend.
+
+Everything above this line works in terms of a Layout and a set of control
+ids and doesn't care what the hardware speaks. Everything below it is one
+pad family's protocol. Adding a family means writing the six methods in
+base.py and listing the class here.
+"""
+
+from .ch57x import CH57x
+from .sayo import Sayo
+
+REGISTRY = (CH57x(), Sayo())   # tried in order; first to claim a device wins
+DEFAULT = REGISTRY[0]
+
+
+def for_vid_pid(vid, pid):
+    for backend in REGISTRY:
+        if backend.claims(str(vid).lower(), str(pid).lower()):
+            return backend
+    return None
+
+
+def for_device(device):
+    # Falls back rather than returning None, so callers that only ever see
+    # one family stay simple. Use for_vid_pid when "nobody owns this" is an
+    # answer you need to act on.
+    if device is None:
+        return DEFAULT
+    return for_vid_pid(device.vid, device.pid) or DEFAULT
+
+
+def known_ids():
+    return [(b.vid, b.pids, b.name) for b in REGISTRY]