Center text on MessageDialogs

Previously MessageDialogs didn't have their text centered, which would result in left-aligned text (visible when the secondary text contains multiple lines). This changes the text to be center aligned, behaving more like libadwaita's MessageDialog.

I'm not sure what the rules are for UI changes like this, but there isn't currently any other way to change the widget's justification, and my use case involves using some behavior not currently present in libadwaita's MessageDialog, so this was much more desirable on my part to mimic what's used there.

Previous behavior:

New behavior: