Dialog: Change default horizontal padding to largeSpacing
We want content to look correctly padded by default, as the zero padded look only works for content that is expected to be scrollable or full-width. If you have dialog content that's normal controls, everything sticks to the edge which looks extremely ugly.
This changes the default horizontal padding to horizontalSpacing, but does not touch the verticalPadding. It's still possible to override the padding to zero like before, and the only difference is if the user of Dialog does not explicitly set padding.