Desktop: add "loading" splash screen while the server is loading

[fmaclen]

It avoids this scenario where the window is created but there is a blank page visible for a second or two, with a lower-case window title:

Here's an example implementation:

export const setLoadingView = (window: BrowserWindow) => {
  if (app.isPackaged) {
    window.loadFile(path.join(process.resourcesPath, 'assets', LOADING_HTML));
  } else {
    window.loadFile(`../../resources/assets/${LOADING_HTML}`);
  }
}

Example:

• Source: https://github.com/fmaclen/canutin-desktop/blob/master/electron/window.ts#L46-L52
• HTML: https://github.com/fmaclen/canutin-desktop/blob/master/resources/assets/loading.html

[fmaclen]

@GregoMac1 I think this one can wait.