x11: add labels to various buttons returned from get_keyboard_map()

This allows identifying, eg. the é button on French keyboard (which is at the location where 2 is on QWERTY)

This is not intended to be complete.  One must still choose what to display depending on the label and the mapped button handle (if any).

panda/src/x11display/x11GraphicsWindow.cxx

@@ -2014,11 +2014,31 @@ get_keyboard_map() const {
 
     KeySym sym = XkbKeycodeToKeysym(_display, k, 0, 0);
     ButtonHandle button = map_button(sym);
-    if (button == ButtonHandle::none()) {
+    std::string label;
+
+    // Compose a label for some keys; I have not yet been able to find an API
+    // that does this effectively.
+    if (sym >= XK_a && sym <= XK_z) {
+      label = toupper((char)sym);
+    }
+    else if (sym >= XK_F1 && sym <= XK_F35) {
+      label = "F" + format_string(sym - XK_F1 + 1);
+    }
+    else if (sym >= XK_exclamdown && sym <= XK_ydiaeresis) {
+      // A latin-1 symbol.  Translate this to the label.
+      char buffer[255];
+      int nbytes = XkbTranslateKeySym(_display, &sym, 0, buffer, 255, 0);
+      if (nbytes > 0) {
+        label.assign(buffer, nbytes);
+      }
+    }
+
+    if (button == ButtonHandle::none() && label.empty()) {
+      // No label and no mapping; this is useless.
       continue;
     }
 
-    map->map_button(raw_button, button);
+    map->map_button(raw_button, button, label);
   }
 
   return map;