To have full control of the backlight level.

Including documentation.

- change API of ps2_mouse; ps2_mouse_task()
- remove mouse_report from host.c

- Add action_util.c and remove action_oneshot.c
- Add oneshot_mods for MODS_ONESHOT
- Add weak_mods for MODS_KEY and MACRO
- weak_mods is cleared when layer switching

- Remove ACT_LAYER_BITOP

- Remove ACT_OVERLAY
- Rename ACT_KEYMAP to ACT_LAYER
- Add ACT_LAYER_BITOP

- Can't use Invert action for tap key, use On/Off insted.

- current_layer indicates active layer at the time
- default_layer indicates base layer
- default_layer is used when current_layer is 0
- with this LAYER_BIT action works as overlay even if
  default_layer varies other than layer 0.