[Feature] Re-Order or Hide Conversion Units

Hello, I just found the app and it has already been a great help in the kitchen. Very handy to do the calculations and then swipe over to conversions. One thing I noticed is that it would be great to have the option of reordering conversion units, so the ones users find most helpful can be placed near the top. Another option might be to have a *Custom* "Physical Quantity" category, where users can set a mix of different conversions to be displayed. ``` grams <-> oz feet <-> meters Tbsp <-> tsp cups <-> mL byte <-> GB tsp <-> mL ```