Add function ReflowVisibleGroupedTBBars

[senowak]

Summary

Grouped Tracking Bars were previously anchored statically based on their configured order. When an earlier grouped bar was hidden because its buff was inactive, later active bars kept their original anchors, leaving empty gaps in the group.

Changes

EllesmereUICdmBuffBars.lua only - dynamic grouped-bar reflow for Tracking Bars.

1. Dynamic reanchor for grouped Tracking Bars
   Added a reflow pass that rebuilds the visible layout of grouped Tracking Bars based on currently shown bars only. The configured bar order is still respected as the hierarchy/source of truth, but inactive or hidden bars are skipped when calculating anchors.

2. Grow Direction aware positioning
   The reflow uses the bar's configured grow direction, so active bars expand in the expected direction instead of always occupying fixed static slots.

Behaviour

Only Buff 2 active → Buff 2 occupies the first visible group position
Buff 2 and Buff 3 active → Buff 3 anchors after Buff 2 according to grow direction
Buff 1 becomes active → Buff 1 takes the first position, Buff 2 and Buff 3 move down/up accordingly
Buff 1 disappears → Buff 2 moves back into the first visible position, removing the gap

Why

This makes grouped Tracking Bars behave like a dynamic active-buff list rather than a fixed layout with hidden placeholders. It preserves user-defined priority/order while keeping the visible group compact.