UI enhancements

[sc4n1a471]

• [x] #91
• [x] Sheet background on MapView
• [x] Buttons on sheets should look like the buttons on the detail sheet in Apple Maps
• [x] Don't show model/type_code on MyCarsView if empty string
• [x] Mileage chart enhancements
• [x] Buttons / Loading indicator improvements on DetailView (replace icons during loading, not hide them, like on QuerySheetView)
• [x] Colored markers on MapView based on unknown/known status
• [x] #102
• [x] #104
• [x] #105
• [x] #106
• [x] #107
• [x] #108
• [x] #110
• [x] #67
• [x] #70
• [x] #7

[sc4n1a471]

For colored markers: https://kristaps.me/blog/swiftui-map-markers