Add is_window() and isnt_window()

[rodo]

Add functions to detect if a function is defined as window function or not. These functions will work same as is_definer() and is_aggregate()

Ref : https://www.postgresql.org/docs/9.1/static/tutorial-window.html

@theory what about the names is_window() and isnt_window() ?

[theory]

Want to make a PR, @rodo?

[rodo]

yes sure, I'll work on it for the 0.99 version !