Fix several warnings spotted by check_layout.

* layouts/latn_bone.xml: use new syntax and add most missing keys

new syntax: n/s/e/w/ne/nw/se/sw instead of key{1..8}

Some keys were missing keys according to the tests, this adds them in a
similar way to latn_neo2.
Only `switch_forward` didn't fit (because of the 0
at the bottom of the spacebar).

* layouts/latn_bone.xml: even better bone

- use number row to unclutter rest of keyboard
- use modmap for special shift functions of bone layout
- add missing loc characters
- use standard bottom bar
- add diacritics from the 3 dead key keys on bone
- 2 small keys left and right of the number row (don't fit all 6
  diacritics, so they leak onto the number keys)
- row 1 is full too, so the dead keys from the left of that row leak
  onto the keys there as well

open questions:
- should bone rely on the math layer or use modmap?
- possibility to combine diacritics?

* shell.nix: fix gradle error

error was:
> \> Task :compileDebugJavaWithJavac FAILED
> error: Source option 7 is no longer supported. Use 8 or later.
> error: Target option 7 is no longer supported. Use 8 or later.

fixed by overriding the jdk version for gradle to openjdk17

* layouts/latn_bone: fix layout

- fix alignment with tabs mess
- make like actual bone layout, instead of trying to fit it into a 10 key
  wide keyboard
  - fixes missing üäö
  - fix missing $
  - moves q and ß where they belong
  - remove all of the diacritic keys (they can be added through the
    settings as extra keys)
  - kept the number row extra keys integration into top row
  - kept the idea of compressing , and . into the swipe actions of the
    bottom row
  - kept the number keys as key4 in the layer4 positions (instead of
    moving them to a number row, which i also considered)

This separates the layout definitions from the special layouts
(bottom_row, greekmath) and other unrelated files (method, settings).

This is also a more intuitive location for layouts and make the resource
directory easier to navigate.

Under the hood, layouts are copied back into
build/generated-resources/xml.