When calling set_checked(true) on an exclusive button, we will now
transfer focus to the newly checked button if one of its now-unchecked
siblings had focus before.
This makes windows that place initial focus somewhere in a group of
radio buttons look nicer when they show up, since focus will be on
whichever radio button was pre-checked, which may not be the first one
in the group.
Making an AbstractButton exclusive means that we enforce that only one
of the exclusive buttons within the same parent widget can be checked
at a time.
RadioButton was doing exactly the same thing, except in a custom way.
So just remove the custom code and make it exclusive. :^)
Every widget now has a GUI::FocusPolicy that determines how it can
receive focus:
- NoFocus: The widget is not focusable (default)
- TabFocus: The widget can be focused using the tab key.
- ClickFocus: The widget can be focused by clicking on it.
- StrongFocus: Both of the above.
For widgets that have a focus proxy, getting/setting the focus policy
will affect the proxy instead.
Instead of everyone overriding save_to() and set_property() and doing
a pretty asymmetric job of implementing the various properties, let's
add a bit of structure here.
Object properties are now represented by a Core::Property. Properties
are registered with a getter and setter (optional) in constructors.
I've added some convenience macros for creating and registering
properties, but this does still feel a bit bulky. We'll have to
iterate on this and see where it goes.
