feat(addons): let an add-on talk to its own full-tab pages

api.emit() only ever reached the sidebar, even though addon-tab-preload has
always exposed silentmode.on() — so an add-on could hear from its panel but
had no way to tell its own editor tab anything. An add-on that wants a
second document to land in the editor already open, instead of a third tab
full of ribbon, needs exactly that.

Two small pieces:

- emitToPanel now also delivers to every tab owned by the same add-on. The
  sidebar keeps its existing condition; tabs are additional, not instead.
- addon-tab-focus, the counterpart of addon-tab-close: a page asking for its
  own tab to be fronted. Needed because the click that hands an open page
  something new usually happens somewhere else — the sidebar — and the
  result would otherwise appear in a tab nobody is looking at.

Both derive the tab from the sender's webContents, the way the close handler
already does, so a page can only front or close the tab it is itself in.

addon-tab-preload.js

@@ -17,4 +17,7 @@ contextBridge.exposeInMainWorld("silentmode", {
   // Escape. Main derives the tab from the sender's webContents id so an
   // add-on page can only close its own tab, never anyone else's.
   closeTab: () => ipcRenderer.invoke("addon-tab-close"),
+  // Bring this page's own tab to the front. Main derives the tab from the
+  // sender, so a page can only front itself.
+  focusTab: () => ipcRenderer.invoke("addon-tab-focus"),
 });