Controlling layer opacity needs better layer labels: which data is layer 1, 2, ...

[aylward]

Layers need more specific labels in the rendering tab - otherwise it is unclear which data relates to which layer.

Ideas for solving this

1. Changer layer name to match data name
2. Each time a layer is added, it is assigned a number and that number is listed on the data thumbnail next to the +/- layer icon. That number is then used when listing layers on the rendering tab.
3. Right clicking or hovering over a name layer listed in the rendering tab causes information on that layer to appear: filename, colormap, show/hide, ...

[PaulHax]

Implemented idea 1 in #479

[PaulHax]

Layer names are matching the data browsers series data name now.