Fix applets inappropriately showing a line or flat border when floating

1. I hide the line that's drawn at the bottom of applets when they are
floating (NOT system tray applets, which already had no line).

2. I make sure applets know that they're meant to be floating by telling
the panel to set the floating flag when the containment changes too; this
is because it is first set when the containment is still `null`, so the
applets didn't get the memo at all.