"Check for update" button in settings

[sjasct]

A "check for application update" would be great for those who know that there is an update but have ignored it before.

[bastimeyer]

When clicking the ignore button, the popup will reappear after 7 days, so there actually is no real need for adding this feature, unless there was a critical bugfix update, etc. But yeah, I can of course add something like that when I'm done implementing the download mechanism for the (Windows) installer. See #145