Timeline: Quick Offset layers and keyframes in selection order (#1012)

Ctrl/Cmd+Alt dragging a selected layer bar or a selected key now runs a
Quick Offset (layer.quickOffset / keys.quickOffset with fromStart, one
undo step). The mode is chosen when the drag starts and needs at least
two unlocked layers; the last selected layer moves by whole frames with
the pointer. A readout by the pointer shows the Total Offset and the
offset Per Layer, translated.

A plain drag still moves the selection together, Alt alone still scales
a key group in time (it now leaves Ctrl/Cmd+Alt to Quick Offset), and
Ctrl/Cmd+Alt+click still toggles Hold. The key drag's readout is drawn
by a shared drag_tip. Keys also register timeline.keyAt.<uid>.<ticks>,
exact where keys on subframes can share a frame.

Tests (ui_quick_offset): staggering layer bars in selection order and
keys per layer, one undo each; plain drags and Alt scaling unchanged; the
readout while dragging.

docs/control-protocol.md

@@ -118,7 +118,8 @@ one), `panel.tab.Viewer(<n>)` and `viewers.<n>` (View ▸ New Viewer's other Com
 renames it), `viewer.comp`, `viewer.handle.<layer>.<i>`,
 `timeline.layer.<id>.bar`, `timeline.layer.<id>.nestedMarker.<i>` (a precomp's comp markers), `timeline.layer.<id>.twirl`, `timeline.layer.<id>.row` (click selects,
 Enter or a double-click renames, drag reorders), `timeline.prop.<uid>.stopwatch`,
-`timeline.key.<uid>.<frame>`, `timeline.cti`, `effectControls.prop.<uid>.value` (an angle's
+`timeline.key.<uid>.<frame>` (and `timeline.keyAt.<uid>.<ticks>`, exact in layer-time ticks, for
+keys on subframes), `timeline.cti`, `effectControls.prop.<uid>.value` (an angle's
 revolutions are `timeline.prop.<uid>.revolutions`, `effectControls.prop.<uid>.revolutions` and
 `properties.prop.<uid>.revolutions`) and `effects.item.<name>`. The Composition viewer adds `viewer.magnification`, `viewer.resolution`,
 `viewer.roi`, `viewer.grid`, `viewer.channel`, `viewer.exposure`, `viewer.snapshot`,