Add layer stack
Add poker keymap with layer stacking.
Change: 0 means default_layer in current_layer now

- 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.
Fix Make dependency file names
Add layer_stack files taking apart from action.c
Add poker keymap with layer stacking
Fix switch_default_layer command
Merge branch 'layer_stack'
Replace layer_stack with layer_switch
Add keymaps of gh60

- keymap_poker.h            poker emulation
- keymap_poker_set.h        poker emulation with Esc fix and cursor fix
- keymap_poker_bit.h        poker emulation with Esc fix and cursor fix
Rewrite layer action with layer_switch
Add keymap file of gh60
Merge branch 'layer_switch'
Apply new keymap for gh60
Add overlay framework
Add keymap clear/reset action
Fix tap key bug: layer stuck

- Can't use Invert action for tap key, use On/Off insted.
Fix keymap of gh60 and hhkb
Add initial files for PC98
Quick Fix: read scan code from PC98