Linux: remove misleading C++-style default values (#253)

Even if `window_manager_plugin.cc` is compiled as C++, C++-style inline
initialization of struct members doesn't work for `_WindowManagerPlugin`
because the instance is not created by the C++ operator new but alloc'd
and zero-filled by the GObject type system.

> GObject explicitly guarantees that all class and instance members
> (except the fields pointing to the parents) to be set to zero.

https://docs.gtk.org/gobject/concepts.html#object-instantiation

Initial values can be set in `window_manager_plugin_init()` if needed.

Co-authored-by: LiJianying <lijy91@foxmail.com>

linux/window_manager_plugin.cc

@@ -18,17 +18,18 @@ struct _WindowManagerPlugin {
   FlPluginRegistrar* registrar;
   FlMethodChannel* channel;
   GdkGeometry window_geometry;
-  GtkWidget* _event_box = nullptr;
-  bool _is_prevent_close = false;
-  bool _is_maximized = false;
-  bool _is_minimized = false;
-  bool _is_fullscreen = false;
-  bool _is_always_on_top = false;
-  bool _is_always_on_bottom = false;
-  bool _is_dragging = false;
-  bool _is_resizing = false;
-  gchar* title_bar_style_ = strdup("normal");
-  GdkEventButton _event_button = GdkEventButton{};
+  GtkWidget* _event_box;
+  bool _is_prevent_close;
+  bool _is_frameless;
+  bool _is_maximized;
+  bool _is_minimized;
+  bool _is_fullscreen;
+  bool _is_always_on_top;
+  bool _is_always_on_bottom;
+  bool _is_dragging;
+  bool _is_resizing;
+  gchar* title_bar_style_;
+  GdkEventButton _event_button;
   GdkDevice* grab_pointer;
   GtkCssProvider* css_provider;
 };