MessageDialog: Make labels not selectable

[leolost2605]

I've personally never needed to select anything in the message dialog labels and I find it kind of annoying/confusing being able to place a cursor there and it being selected by default sometimes (I think e.g. in sideload). Not sure though, just my opinion 🤷

[danirabbit]

The original intent of making the labels selectable was so someone could copy an error message to search for it in case they were confused. Perhaps a better solution is to set the focus_widget to the action area's last child?