Add P{B,F,N,S}_CASE & LUAKIT_WIDGET_{,BIN_}INDEX_COMMON macros

Add P{B,F,N,S}_CASE & LUAKIT_WIDGET_{,BIN_}INDEX_COMMON macros

The cases:
  PB_CASE(tok, bool) -> push a boolean on to the lua stack and return
  PF_CASE(tok, func) -> push a function on to the lua stack and return
  PN_CASE(tok, num)  -> push a number on to the lua stack and return
  PS_CASE(tok, str)  -> push a string on to the lua stack and return

LUAKIT_WIDGET_INDEX_COMMON:
  Class method cases common to all gtk widgets

LUAKIT_WIDGET_BIN_INDEX_COMMON:
  Class method cases common to all gtk bin widgets