YaruTitleBar: trailing -> actions

[jpnurmi]

To make YaruTitleBar closer to a drop-in replacement of AppBar.

Close: #515

[jpnurmi]

Hmm, I noticed that AppBar has an interesting check for determining whether to center the title depending on the amount of actions. I wonder if we should have something similar... 🤔

  bool _getEffectiveCenterTitle(ThemeData theme) {
    bool platformCenter() {
      assert(theme.platform != null);
      switch (theme.platform) {
        case TargetPlatform.android:
        case TargetPlatform.fuchsia:
        case TargetPlatform.linux:
        case TargetPlatform.windows:
          return false;
        case TargetPlatform.iOS:
        case TargetPlatform.macOS:
          return actions == null || actions!.length < 2;
      }
    }

    return centerTitle
      ?? theme.appBarTheme.centerTitle
      ?? platformCenter();
  }

[Feichtmeier]

Still we mimic the "api" here in most places: sounds good

[jpnurmi]

Sure, it was just something that caught my attention while checking how AppBar handles actions. I think this change is fine regardless of what we decide to do with centerTitle.