Add the ITPROTECH MK series (8808) as a third pad family

Confirmed on the MK-09 (8808:660a, 9 keys): the vendor's .NET configurator
was decompiled for the wire format and every frame was then written to a
real pad and read back. Keys, media keys and multikey sequences are
supported; lighting from the command line.

The firmware acts on every other output report, so each report is sent
twice (as the vendor software does) and every write is verified by reading
the key back. The app now keeps one state file per pad and starts with a
known layout for a known USB id, and the editor asks the backend whether a
binding fits before offering to write it. The actions listener and the
udev rule cover every supported vendor.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

macropad_gui/backends/base.py

@@ -15,6 +15,7 @@ class Backend:
     layers = 1
     max_keys = 0
     max_knobs = 0
+    sequences = False     # can one control send several keys in a row?
 
     def claims(self, vid, pid):
         """True only for ids this backend has been confirmed on."""
@@ -40,6 +41,13 @@ class Backend:
     def supports(self, keys, knobs):
         return 1 <= keys <= self.max_keys and 0 <= knobs <= self.max_knobs
 
+    def check(self, binding):
+        """
+        Why this family can't hold a binding the GUI thinks is valid, or
+        None. Lets the editor say so while typing instead of at write time.
+        """
+        return None
+
     # The six a family must answer.
 
     def action_byte(self, control):