YaruTitleBar: make themable and use window controls

[jpnurmi]

Visual changes extracted from #455, leaving out any actual top-level window-related functionality i.e. YaruWindowTitleBar, YaruDialogTitleBar, and YaruWindowController.

NOTE: This is a breaking change. YaruTitleBar no longer creates a YaruCloseButton that calls Navigator.pop. Instead, it has a set of properties that can be used to manage which YaruWindowControls to create.

P.S. YaruWindowTitleBar and YaruDialogTitleBar are still coming but without window_manager dependency as originally proposed in #455. :)

[Feichtmeier]

Cool, could you add some .close() constructor that adds a close button by default?

[jpnurmi]

There's a separate YaruDialogTitleBar in the works that will call Navigator.pop out of the box. With the "plain" YaruTitleBar you can do:

YaruTitleBar(
  isClosable: true,
  onClose: Navigator.of(context).pop,
)