This patch removes size policies and preferred sizes, and replaces them
with min-size and max-size for each widget.
Box layout now works in 3 passes:
1) Set all items (widgets/spacers) to their min-size
2) Distribute remaining space evenly, respecting max-size
3) Place widgets one after the other, adding spacing in between
I've also added convenience helpers for setting a fixed size (which is
the same as setting min-size and max-size to the same value.)
This significantly reduces the verbosity of widget layout and makes GML
a bit more pleasant to write, too. :^)
Since the vast majority of message boxes should be modal, require
the parent window to be passed in, which can be nullptr for the
rare case that they don't. By it being the first argument, the
default arguments also don't need to be explicitly stated in most
cases, and it encourages passing in a parent window handle.
Fix up several message boxes that should have been modal.
Now that add() returns a WidgetType&, we can't rely on the parent of a
GUI::Dialog to still keep it alive after exec() returns. This happens
because exec() will call remove_from_parent() on itself before
returning.
And so we go back to the old idiom for creating a GUI::Dialog centered
above a specific window. Just call GUI::Dialog::construct(), passing
the "parent" window as the last parameter.
Since the returned object is now owned by the callee object, we can
simply vend a ChildType&. This allows us to use "." instead of "->"
at the call site, which is quite nice. :^)
