Display hotkey in toolbar button's tooltip on Australis

[Infocatcher]

Built-in buttons:
resource://app/modules/CustomizableWidgets.jsm

    id: "privatebrowsing-button",
    shortcutId: "key_privatebrowsing",

function setAttributes(aNode, aAttrs) {
  ...
        let additionalArgs = [];
        if (aAttrs.shortcutId) {
          let shortcut = doc.getElementById(aAttrs.shortcutId);
          if (doc) {
            additionalArgs.push(ShortcutUtils.prettifyShortcut(shortcut));
          }
        }
        value = CustomizableUI.getLocalizedProperty({id: aAttrs.id}, stringId, additionalArgs);
      }
      aNode.setAttribute(name, value);
  ...
}

resource://app/modules/CustomizableUI.jsm

  getLocalizedProperty: function(aWidget, aProp, aFormatArgs, aDef) {
    ...
    try {
      if (Array.isArray(aFormatArgs) && aFormatArgs.length) {
        return gWidgetsBundle.formatStringFromName(name, aFormatArgs,
          aFormatArgs.length) || def;
      }
      return gWidgetsBundle.GetStringFromName(name) || def;
    } catch(ex) {
      if (!def) {
        ERROR("Could not localize property '" + name + "'.");
      }
    }
    ...
  },

Looks like we can't use this way, but now we have the main part: ShortcutUtils.prettifyShortcut().

[Infocatcher]

And yes, we have Ctrl+Alt+P in menu and Alt+Ctrl+P in tooltip.
Don't ask me, why, I just used API: key="idOfXULKeyNode" in menu and built-in ShortcutUtils.prettifyShortcut(XULKeyNode) to create tooltip.