Redesign panel settings with a dialog

This moves all panel options in a single dialog that floats near the panel.

BUG: 450892
BUG: 459103
BUG: 458008
BUG: 440274
BUG: 453693
BUG: 460844
BUG: 455478
BUG: 462980
BUG: 454720
BUG: 455476
BUG: 460845
BUG: 468009
BUG: 474354
BUG: 364456
FIXED-IN: 6.0