PortalDialog: Fix various padding and sizing issues

This is especially noticeable in the "Remote control requested" dialog, the padding between items in the header is wonky and icons tend to be too large for its text.

I shrunk the size of the icon which better fits our heading, and also increased the spacing between them. I also found that the spacing between the header item and the window was wrong, it needed to be largeSpacing according to our HIG. The result is a more balanced-feeling window.

Before	After