Check for Update duplicated

[pljones]

Checking whether there is a new version available is done in two places:

• Client Dialogue
• Server Dialogue

with separate code to do the necessary network lookups.

1. There should only be one method implementing the check
2. The check should be unaware of whether it is running from a UI
3. Even when not running a UI, the user should be informed of a new version being available
4. When running a UI, they should be given the option to download and run the installer, exiting the program, if a new version is available

(Originally a comment in #3556 (comment))