This updates CustomUiPreview to look like the new welcome screen. This
also adds new customization features, such as the color and transparency
of the tip boxes and main box. It also adds a default settings button.
https://git.jami.net/savoirfairelinux/jami-client-qt/-/issues/1097#note_44258

Change-Id: I6054dbfa55193e92eac7905e381b7fcdaa44d67e