Cleanup the dialog prompt asking if the user wants to search for updates

This is a cleanup of the dialog and the behavior surrounding it. Specifically, the dialog now looks like this:

(changed the text string to be clearer and switched to a single button that shows it applies the settings and you can't cancel it at all.

I also modified where the update code is getting run. Previously it was inside an idle handler and was guarded by a completely different flag (for if the old windows should be opened). Instead, I have moved them to a function that gets called after the frame is created by adding a CallAfter to the constructor.

@stambaughw I forget, is this an acceptable use of the CallAfter in our code, or will it break the frame creation? It seems to work on my machine at least. Also, is this dialog/string change acceptable for 8.0 now, or should it wait for 8.0.1? (I think getting it into 8.0 would be best, because this dialog is seen by new users, so making sure it is correct and understandable should be a priority).

CC @mroszko I think this should still do the update checks properly, but can you verify/comment on if the KiCad update check here will mess up the processing?