Fix notifications broken since Nativefier 43 / Electron 12 defaulting to contextIsolation:true (PR #1308)

Copy-pastaing details from [Electron 12 breaking changes](https://www.electronjs.org/docs/latest/breaking-changes#planned-breaking-api-changes-120):

> ### Default Changed: `contextIsolation` defaults to `true`[​](https://www.electronjs.org/docs/latest/breaking-changes#default-changed-contextisolation-defaults-to-true "Direct link to heading")
> 
> In Electron 12, `contextIsolation` will be enabled by default. To restore the previous behavior, `contextIsolation: false` must be specified in WebPreferences.
> 
> We [recommend having contextIsolation enabled](https://www.electronjs.org/docs/latest/tutorial/security#3-enable-context-isolation-for-remote-content) for the security of your application.
> 
> Another implication is that `require()` cannot be used in the renderer process unless `nodeIntegration` is `true` and `contextIsolation` is `false`.
> 
> For more details see: [https://github.com/electron/electron/issues/23506](https://github.com/electron/electron/issues/23506)

I find the security drop acceptable, as reverting the new Electron 12 isolation brings us to the previous level of security, and I don't have the time/will to keep the isolation and migrate to the newer better safer thing that Electron >= 12 wants.

Co-authored-by: Radomír Polách <rp@t4d.cz>

app/src/helpers/windowHelpers.ts

@@ -148,6 +148,7 @@ export function getDefaultWindowOptions(
       plugins: true,
       webSecurity: !options.insecure,
       zoomFactor: options.zoom,
+      contextIsolation: false,
       ...webPreferences,
     },
     ...browserwindowOptions,