Replace our custom map implementation with walkers crate

[KoffeinFlummi]

Replaces our hacked-together map implementation with one based on the walkers crate.

Pros:

• actually correct map rendering
• less code for us to maintain
• nice position labels
• another tile provider beside mapbox, useful e.g. if no mapbox token is configured

Cons:

• still requires some upstream changes

Upstream PRs/changes:

• Mapbox tiles (Add Mapbox tile provider podusowski/walkers#76, merged)
• Fixes (Two small fixes podusowski/walkers#79)
• Cache Storage (Support disk cache for tile bitmaps podusowski/walkers#64, Add TileCache trait, incl. disk & noop implementations podusowski/walkers#82)
• Path Drawing (could also be implemented as a plugin in Sam)

[podusowski]

Drop a word if you need some changes to be published on the cargo.io.

Cool project btw! :)

[KoffeinFlummi]

Thanks, will do! Depending on time availability, I might get around to another PR or two soonish.