…eaks

This commit is the followup to a similar one I did regarding window initialization. Instead
of widgets being created on the heap and not being managed by a smart pointer until they're
added to a grid, they are now always managed by a shared_ptr. To that end, this commit covers
bunch of things:

* builder_widget::build (both overloads) and all its overrides (which have now been marked
  as such) now return shared_ptr<widget>.
* The builder_grid::build() override now returns the same instead of grid* since you can't
  use covariant return types with smart pointers.
* The two implementation build helpers in builder_grid have been combined with an optional
  replacement map as the second parameter. Uses of the version that took a grid pointer could
  be easily converted to pass a reference instead.
* The pane, matrix, and viewport build() functions were removed in favor of making the ctor
  public. In case there was a deprecated ctor, that was removed.
* The viewport now keeps a widget shared_ptr instead of a reference that was then deleted
  by address-of. This was both better design and necessary to fix a crash.