Introduce automatic update check for Gajim
This adds an automatic update check for Gajim when run on Windows/MacOS.
On first start, Gajim will ask the user if it should check for updates periodically (every 7 days). If automatic update is enabled and a new version is advertised on https://gajim.org/current-version.json (not published yet), Gajim will show a dialog offering to open https://gajim.org/download
This dialog also offers a checkbox to disable automatic update checking. Additionally, there is a new checkbox in Preferences > Advanced to enable/disable automatic update checks.
Fixes #5315 (closed)