[macOS,Windows] Screen sharing takes too long to generate a thumbnail, causing UI blocking.

[cloudwebrtc]

When getting the enumerated window/screen source, the synchronous mode is used when getting the thumbnail, causing the first call to getSources to block for a long time when there are 10 or more windows

Solutions:

1. When enumerating sources for the first time, only id, title, and type are returned.
   A blank image placeholder is displayed on the UI
2. Create an asynchronous thread to intercept thumbnails, compress thumbnails,
   Emit desktopSourceThumbnailChanged to update to the latest thumbnail