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/base.py

@@ -0,0 +1,64 @@
+"""
+What a device family has to provide.
+
+These raise rather than returning something vague, so a half written backend
+fails at the call it can't answer instead of writing wrong bytes to somebody's
+hardware.
+"""
+
+
+class Backend:
+    key = ""              # short id, used in settings and messages
+    name = ""             # what a person would call this family
+    vid = ""              # USB vendor id, lowercase hex, no 0x
+    pids = ()             # product ids this backend is CONFIRMED on
+    layers = 1
+    max_keys = 0
+    max_knobs = 0
+
+    def claims(self, vid, pid):
+        """True only for ids this backend has been confirmed on."""
+        if vid.lower() != self.vid.lower():
+            return False
+        return not self.pids or pid.lower() in {p.lower() for p in self.pids}
+
+    def could_claim(self, vid, pid):
+        # Siblings share a vendor id but not always a protocol. The scan and
+        # the udev rule cover the whole vendor so an unconfirmed sibling is
+        # still found and offered detection; claims() stays strict so it is
+        # never written to on the strength of its vendor id alone.
+        return vid.lower() == self.vid.lower()
+
+    def udev_rule(self):
+        return (f'KERNEL=="hidraw*", ATTRS{{idVendor}}=="{self.vid}", '
+                f'TAG+="uaccess"')
+
+    def adopt(self, layout):
+        """Where a family can say more about a shape than detection found."""
+        return layout
+
+    def supports(self, keys, knobs):
+        return 1 <= keys <= self.max_keys and 0 <= knobs <= self.max_knobs
+
+    # The six a family must answer.
+
+    def action_byte(self, control):
+        """The slot number this family uses for a control id."""
+        raise NotImplementedError
+
+    def info(self, device):
+        """(keys, knobs) as the pad reports them, or None."""
+        raise NotImplementedError
+
+    def read_layer(self, device, layer=1, layout=None):
+        raise NotImplementedError
+
+    def write(self, device, control, binding, state, layer=1, _write=None):
+        raise NotImplementedError
+
+    def detect(self, device):
+        """Read only. Must say no rather than guess."""
+        raise NotImplementedError
+
+    def __repr__(self):
+        return f"<{type(self).__name__} {self.vid}:{'/'.join(self.pids) or '*'}>"