gs-updates-page: Keep showing apps which are being installed
The idea is that a user enters the Updates page and sees one Flatpak application for update, thus he/she click "Update" and the application switches to the "installing" state and the user can watch the progress of the install. While the page is shown it can either refresh on its own or after user's intent to refresh the content. When it's done the page shows empty with "System is up to date", which is considered a lie, because there is still an ongoing update. This can be verified in the Installed page, where the application is shown as "installing".
Rather than presenting the user suddenly empty Updates page, I suggest to show there also applications, which are installing.