issue #23319 - client update

Fix the reported bug #23319: Check installation destination for write privs and `sudo` if necessary (Mac and Linux)
- Download to a temp dir instead of the current directory (in Qt 5 we'll be able to ask for the user's downloads dir)
- Unpack the bundles created by our build scripts instead of requiring conversion for automatic updates

These changes do _not_ address the download host issue, where the bundles have to be copied to a special location so the automatic updates can find them.