css: Add -gtk-dpi CSS property hack

css: Add -gtk-dpi CSS property hack

This property is necessary to ensure widgets automatically update after
the text scale factor is changed desktop-wide.

And if I'm already doing a property like this, I can make it
overridable. So now you can override the dpi per-widget with CSS like
GtkSwitch {
  -gtk-dpi: 48;
}
if you want to debug things.

Long-term, we want to get rid of this property and insist on dpi being
96 everywhere and people can change the font size to get larger fonts.