[Keymap] Updated layout after some months of testing. (#5427)

[Keymap] Updated layout after some months of testing. (#5427)

* Updated layout after some months of testing.

The current setup looks quite better than the initial one. It's not
perfect but it allows me to work quite well.

The only big problem is the Alt modifiers on the home row which
sometimes break the typing if keys are pressed too fast. I'm also not
satisfied by the TAB key position on the thumb cluster.

Main improvements since previous push:
- Shift layouts on both halves
- Moved around symbols
- Improved number/fn keys
- Reorganized thumbs

* Used enum as suggested by @drashna :-)