* Don't default heap mempool by default now that there's a dedicated
LVGL mempool
* Set proper defaults for CiZ display hardware/custom screen.
* Double the dedicated display thread stack size for CiZ.
* Previous version of multiple overrides of the default value of ZMK_USB
were problematic. Move to using board _defconfig files for the
defaults for those, along with proper `depends on` for ZMK_USB that
accounts for split and split roles.
* LV_ prefix from new LVGL official Kconfig now used.
* API changes for themes, container object removal, etc.
* Add our own Kconfig and code for theme default small font.
* Remove some hardcoded sizes.
