Feature: Upgrade planner

[jjspace]

One of my favorite parts of the old Tank gear spreadsheets was the Upgrade Evaluator that let you compare 2 gearsets and see which items you still need to get and their cost. It was always super invaluable for me early in every tier in EW in planning how and what to spend tomes and books on. (especially when tracking an alt too) It's also one of the biggest features that is completely missing from xivgear and from Etro that make me miss the old spreadsheets the most.

I think it would be really nice to include in xivgear and fill a big void in functionality in the community right now. Comparing stats between sets is great, but when it comes to actually comparing the items it's tedious to switch back and forth between gearsets (or tabs with etro).

This would be somewhat related to #262 and #141 but with less focus on showing all the materia and stats of each set and a very strong focus on the actual difference in items and the cost of the items that you've yet to obtain.

I think a first version of this could be a modal that just shows 2 dropdowns at the top to select gearsets then the list of their items below that with arrows for ones that are different (like above) and a cost column on the right with a total below it. Further improvements could be to display the stats (or the stat delta) and potentially the time it would take to get the books/tomes needed like in the spreadsheets.

I was thinking of trying to work on this myself but wanted to open an issue for discussion in before putting in a ton of work in case it's not a desired feature or you have better ideas for implementation.