[Keymap] the results of some experiments and radically simplify keymap.c (#6172)

* keymap simplification and fancy alt tab behaviour

* move symbols around and try ergo numbers

* mess with symbol positions

* f11 and f12 for volume control (for ease of remapping)

* slack unread navigation

* experiment with mods on home row

* mods on symbol layer

* dedicated tab left and tab right keys

* swap next and prev

* remove hold to shift on a and o

* revert to simpler keymap

* restore readme

* point to keymap image

* cmd + cmd -> cmd + ctrl

* expand readme

* slack unread channel navigation

* Update keyboards/planck/keymaps/callum/keymap.c

Co-Authored-By: Drashna Jaelre <drashna@live.com>
* return true from cmd handling block