Make text selection stylesheetable

Make text selection stylesheetable

This adds a new [label="selected"] that you can use in stylesheets to determine how
a text selection should look like in ChatView (i.e. it's no longer hardcoded to the
"highlight" and "highlighted-text" Palette roles).

Note that you probably shouldn't do more than setting the selection colors globally for
ChatLine, as else the painting routines might do funky stuff.