Set parent window in About dialog
Fixes dialog text not always fitting in screen. Additionally, the dialog is now centered instead of appearing at the bottom of the screen.
Added extra commit to update the website URL.
It still does not look perfect. Probably the easiest way to make it look like other dialogs would be to depend on libadwaita and use an AdwApplication instead of a GtkApplication. I thought that would be a bit too much, so I just added a parent window.