This is a work-in-progress to create a consistent API and finally
separate the whole Aseprite base/gfx/ui libs into a reusable C++ library.
Classes:
app::IFileItem, app::AppMenuItem, app::skin::SkinPart,
gfx::Rect, gfx::Border, she::FileDialog,
ui::IButtonIcon, ui::Graphics, ui::Overlay, ui::Widget,
ui::ScrollableViewDelegate, and UI events