Propagate "update check" prompt to UI checkbox

[deeplow]

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

[deeplow]

@apyrgio I was not sure what the best state propagation method was. I opted for sending a "refresh" signal, but the state is written to and read from the settings.

We could do that through the signal and it would also be fine, especially since this is an edge case and we don't need to keep track of that propagated state (like the docs_added we had in the main_window a while ago).

[deeplow]

@apyrgio that's way more elegant. Let's go with that. I haven't made a test, though since I didn't find a decent place to go a test for this. Since the behavior only really happens in gui/__init__.py. Do you have suggestions regarding testing this? Or should we leave it untested?

[apyrgio]

Hm, you're right, there's no easy way to test it. Given that it's a one line thing, maybe we can just test it manually.

[deeplow]

Anything else missing @apyrgio?

[apyrgio]

Nope, just checked it manually as well, and it seems to work. Feel free to merge it with just the latest commit.