Add ability to show breakdown across all Packs instead of just current Pack

[karlding]

While the "Pack Weights" table on the left is useful for seeing the total weights for each Pack, it would be nice when clicking "View breakdown" (or some other button/text) to see a pie chart of all items aggregated/flattened across all Packs. Currently it seems like the only breakdown is per-Pack, and there is no way to aggregate/"flatten" the breakdown across all Packs being brought on the trip?

One example where this might be useful is for bike-packing. When bike-packing, you might have several "packs" that your gear is split across for better weight distribution–panniers, frame bags, handlebar bags, seatpost bags, etc. Treating each pannier/bag as a separate Pack lets one make sure that they have everything in that Pack when they leave home.

The "Pack Weights" table allows someone to see how the gear is split across the different bags (ex. panniers on the drive/non-drive side of my bike). However, when separating gear into different Pack tabs, it becomes difficult to see the gear % weight split (ex. to see how much my sleep system contributes to the whole gear weight I'm bringing on a trip, and not just for my "Right Pannier" pack).

If someone wants to see this information, it seems like as a workaround they either need to:

1. Have everything in a single pack, and don't bother indicating which pannier things are in: This loses the ability to check which item is in which Pack
2. Have everything in a single pack, and abuse "Category" variants for each bag so that they're split into a separate section: This makes inventory management difficult if you reuse the gear for non-bike-packing, since there'll be basically multiple versions of the same gear
3. duplicate everything into a separate "flattened" Pack that contains everything which is used for Weight %: Then this messes with Total Pack Weights, unless some "flattened" view tab is provided over all Packs that doesn't get included in the total Pack Weights.