Some construction and terrain tweaks

Can fill spiked pits(t_pit_spiked) to shallow pit
Boarding up empty window will create "t_window_boarded_noglass" instead of "t_window_boarded"
Walkway(t_bridge) gets "flat" flag
Breaking boarded window will create some glass shards
Empty window won't have "barricadable_window" flag anymore
You can board up empty window frame also (to "t_window_boarded_noglass")

New terrain : "t_window_boarded_noglass"
-will not create glass shards on breaking
-will become empty window instead of window frame
Merge pull request #5166 from talktips/construction

Fillable spiked pits and boarded up windows tweaks