Electron app loading improvements + Windows paths fix (#429)

- Electron no longer allows Windows paths in URLs
(https://www.electronjs.org/docs/latest/breaking-changes#behavior-changed-custom-protocol-url-handling-on-windows),
fixes with help of wabac.js to replace `C:\path\to\file` with
`C//path/to/file`
- Electron app cleanup: replace deprecated protocol handlers with new
protocol.handle() method, update types
- Electron app: ensure just filename is displayed on index page,
consistent with other platforms
- Update to wabac.js 2.23.2
- Update to Electron 36.3.2
- Fixes #427
- bump to 2.3.12

tests/embeds.spec.js

@@ -57,7 +57,6 @@ test("same-domain iframe embed is loading", async ({ page }) => {
   const inside = res.frameLocator("iframe").locator("body");
 
   await expect(inside).toContainText("Inside iframe");
-
 });
 
 test("cross-domain iframe embed is loading", async ({ page }) => {
@@ -75,7 +74,6 @@ test("cross-domain iframe embed is loading", async ({ page }) => {
   const inside = res.frameLocator("iframe").locator("body");
 
   await expect(inside).toContainText("Inside iframe");
-
 });
 
 test("sandbox + cross-domain embed is loading", async ({ page }) => {