sticky key support

* added sticky key style layer functions kbfun_layer_sticky_N()
* small bug fix for out of bounds array access on kbfun_layer_push_10() and kbfun_layer_pop_10() in basic.c

The kbfun_layer_sticky_N() function gives similar behavior to sticky keys for modifiers
available on most operating systems. It is considered an accessibility
feature because it alleviates the user from having to hold down
modifiers while pressing a key to produce the modified key function. It
is useful for fast touch typing because you can avoid chording motions
which both strain your hands and take your hands out of home-row
position while pressing normal alpha keys.

This function emulates the 3-state behavior which is default on OS X
and optional in Windows where the modifier cycles between
Off->Once->Locked states. This is particularly handy for symbol layers
where you typically only type one symbol before you want to return to
unmodified typing (layer 0), e.g. 'if (condition) { a = "b" + "c"; }'.
If you assign a symbol layer to a thumb key as a layer sticky cycle,
you can type the entire line of code without taking your hands out of
home row position, holding any modifier keys while keying in
alpha-numeric keys, and you do not need to toggle off the layer after
each symbol is pressed, only immediately before keying the symbol.

The exact behavior of the layer sticky cycle function is defined as
follows for each state:
1) One time down (set on key press) - The layer was not active and the
key has been pressed but not yet released. The layer is pushed in the
one time down state.
2) One time up (set on key release) - The layer was active when the
layer sticky key was released. If a key on this layer (not set to
transparent) was pressed before the key was released, the layer will be
popped. If a non-transparent key was not pressed, the layer is popped
and pushed again in the one time up state.
3) Locked (set on key press) - The layer was active and in the one time
up state when the layer sticky key was pressed again. The layer will be
popped if the function is invoked on a subsequent keypress.