UpdateCheck mechanism should allow to also directly install updates

Phenomenon

To finalize the UpdateCheck mechanism it should be possible to not only download the latest update archive directly. It should also be possible to install the update right away seeing YAM being closed, the updated files/binaries updated and then YAM being restarted to finalize the update.

Background analysis

While for a simple updatecheck mechanism downloading the update lha archive might be enough a better approach would be if the update could also be automatically installed instead of just downloading the archive and relying on the user to install the update properly. Several major problems with that approach seem to prevent the implementation however:

1. We have to find a way to close YAM remotely (e.g. via ARexx) and restart it after the files from the update archive were installed.
2. We have to standardize the update archive to also be able to handle larger update jumps (e.g. if a YAM 2.8 user want to update to YAM 3.0 directly without having to install 2.9 first)
3. We have to take care not to overwrite files that have been changed by the user (e.g. by using md5sum or similar)
4. How should third-party updates (e.g. mcc classes, etc.) be treated?

Implementation recommendation

An ideal implementation would present an "Install Update" button instead of "Download" in the update notification window. If the user presses this button YAM will be closed, the update installed (possibility non-interactive, with creating backups, etc.) and afterwards YAM being restarted to finalize the update.