window: Simplify API for full maximization
The current API covers both full and partially maximization, which makes the former (much more common) case more complex.
Address this by moving the existing API to
get_maximize_flags()set_maximize_flags()set_unmaximize_flags()
and add a set of simpler methods for the full maximization case:
is_maximized()maximize()unmaximize()
See !4413 (comment 2421185) for the initial discussion.