timeline: snap pointer and import operations to timebase

If frame rates are mixed, ensures that video doesn't end up falling or ending on half frames which potentially make the entire sequence awkward

app/widget/timelinewidget/tool/tool.cpp

@@ -53,8 +53,22 @@ Timeline::MovementMode TimelineTool::FlipTrimMode(const Timeline::MovementMode &
   return trim_mode;
 }
 
+rational TimelineTool::SnapMovementToTimebase(const rational &start, rational movement, const rational &timebase)
+{
+  rational proposed_position = start + movement;
+  rational snapped = Timecode::snap_time_to_timebase(proposed_position, timebase);
+
+  if (proposed_position != snapped) {
+    movement += snapped - proposed_position;
+  }
+
+  return movement;
+}
+
 rational TimelineTool::ValidateTimeMovement(rational movement)
 {
+  bool first_ghost = true;
+
   foreach (TimelineViewGhostItem* ghost, parent()->GetGhostItems()) {
     if (ghost->GetMode() != Timeline::kMove) {
       continue;
@@ -63,6 +77,11 @@ rational TimelineTool::ValidateTimeMovement(rational movement)
     // Prevents any ghosts from going below 0:00:00 time
     if (ghost->GetIn() + movement < 0) {
       movement = -ghost->GetIn();
+    } else if (first_ghost) {
+      // Ensure ghost is snapped to a grid
+      movement = SnapMovementToTimebase(ghost->GetIn(), movement, parent()->GetTimebaseForTrackType(ghost->GetTrack().type()));
+
+      first_ghost = false;
     }
   }