Send NumLock on macOS, even though the key is Clear

There is no obvious choice what works best here, but this is what
TigerVNC has been doing for years without complaints. Let's follow
them until we get reports that this doesn't work well.

kasmweb/core/input/util.js

@@ -156,6 +156,14 @@ export function getKeysym(evt) {
             location = 2;
         }
 
+        // And for Clear
+        if ((key === 'Clear') && (location === 3)) {
+            let code = getKeycode(evt);
+            if (code === 'NumLock') {
+                location = 0;
+            }
+        }
+
         if ((location === undefined) || (location > 3)) {
             location = 0;
         }
@@ -172,6 +180,15 @@ export function getKeysym(evt) {
             }
         }
 
+        // macOS has Clear instead of NumLock, but the remote system is
+        // probably not macOS, so lying here is probably best...
+        if (key === 'Clear') {
+            let code = getKeycode(evt);
+            if (code === 'NumLock') {
+                return KeyTable.XK_Num_Lock;
+            }
+        }
+
         return DOMKeyTable[key][location];
     }