Problem:
- Many constructors are defined as `{}` rather than using the ` =
default` compiler-provided constructor.
- Some types provide an implicit conversion operator from `nullptr_t`
instead of requiring the caller to default construct. This violates
the C++ Core Guidelines suggestion to declare single-argument
constructors explicit
(https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c46-by-default-declare-single-argument-constructors-explicit).
Solution:
- Change default constructors to use the compiler-provided default
constructor.
- Remove implicit conversion operators from `nullptr_t` and change
usage to enforce type consistency without conversion.
We were relying on Core::is<Window>() to tell us whether the parent
of an action is a window. This didn't work since we only saw a forward
declaration of GUI::Window in Action.cpp.
This is an unfortunate flaw in the is<T> pattern and we should solve
it somehow but not in this patch.
During app teardown, the Application object may be destroyed before
something else, and so having Application::the() return a reference was
obscuring the truth about its lifetime.
This patch makes the API more honest by returning a pointer. While
this makes call sites look a bit more sketchy, do note that the global
Application pointer only becomes null during app teardown.
This patch adds GUI::Action::create_checkable() helpers that work just
like the existing create() helpers, but the actions become checkable(!)
Clients are no longer required to manage the checked state of their
actions manually, but instead they will be checked/unchecked as needed
by GUI::Action itself before the activation hook is fired.
