Adding some special keys to blueshift
Ditch shift sticker and add Fkey layer
Move Plover smarts into handler

Instead of relying on layers. The handler can figure out whether the
layer is active and act appropriately.
Make Fkey layer work with double-layer invocation

The problem with launching off two keys is that they may be misordered,
which leaves the state inconsistent. Fixed with a little smarts in the
handler.