The "check for updates" button wasn't showing up immediately as checked
as soon as the user is prompted for checking updates. This fixes that.
Fixes#513
Fully test the update check logic, by introducing several Qt tests.
Also, improve the `UpdaterThread.get_letest_info()` method, that gets
the latest version and changelog from GitHub, with several checks.
These checks are also tested in our newly added tests.
We want to differentiate between the user clicking on "Cancel" and
clicking on "X", since in the second case, we want to remind them again
on the next run.
