Blender Git Loki

Fix weird placement of "Motion Tracking" in 3D View overlay popup

There was a weird looking gap between the checkbox and the "Motion Tracking"
label. Plus, the label could not be clicked to change the value, unlike
usually.

Issue is that the row is actually a sub-panel header. The checkbox being drawn
with the draw_header() callback, and the label being added as separate item by
the popover panel code. This adds a hack so the checkbox can add the panel
label itself (the popup drawing skips adding the label then). That addresses
mentioned issues.