On the updates tab, show how long it's been since all repositories were refreshed

[NoahAndrews]

Right now, if Foxy Droid doesn't know about any available updates, the Updates tab will display All applications up to date, no matter how long it's been since the repositories have been refreshed. This wording incorrectly implies that the application knows that the applications are up to date, when that's not the case. Instead, the Updates tab should display how long it's been since all repositories were last refreshed.

Like I described in this comment on issue #42, the Software Manager app (source code) for the upcoming REV Robotics Driver Hub is based on Foxy Droid. I've implemented this functionality in that app, in this commit.

I'm not planning on preparing a PR for this feature to Foxy Droid, but I'm more than happy to answer questions about my code from anyone who would like to do that, so just let me know!