Allow keycodes in layers in addition to modifiers

[insel-maz]

Another approach to solving #926.

This change adds the ability to use regular key names in addition to modifiers for layers.

Example: Toggles W or Shift+W when holding down the right control key:

[ids]
*

[main]
rightcontrol = layer(ctrl_layer)

[ctrl_layer]
w = toggle(key_w)

[ctrl_layer+shift]
w = toggle(key_shift_w)

[key_w:w]

[key_shift_w:S-w]

It is even possible to use multiple key names, e.g. S-w-a.