Allow creating views from an external engine

[robert-ancell]

While only one view works at this point in the future this will allow a runner to create multiple views.

To make use of this the runner template would have the following changes:

diff --git a/linux/my_application.cc b/linux/my_application.cc
index 7083591..e56f6d8 100644
--- a/linux/my_application.cc
+++ b/linux/my_application.cc
@@ -10,6 +10,8 @@
 struct _MyApplication {
   GtkApplication parent_instance;
   char** dart_entrypoint_arguments;
+
+  FlEngine* engine;
 };
 
 G_DEFINE_TYPE(MyApplication, my_application, GTK_TYPE_APPLICATION)
@@ -53,7 +55,9 @@ static void my_application_activate(GApplication* application) {
   g_autoptr(FlDartProject) project = fl_dart_project_new();
   fl_dart_project_set_dart_entrypoint_arguments(project, self->dart_entrypoint_arguments);
 
-  FlView* view = fl_view_new(project);
+  self->engine = fl_engine_new(project);
+
+  FlView* view = fl_view_new_for_engine(self->engine);
   gtk_widget_show(GTK_WIDGET(view));
   gtk_container_add(GTK_CONTAINER(window), GTK_WIDGET(view));
 
@@ -103,6 +107,7 @@ static void my_application_shutdown(GApplication* application) {
 static void my_application_dispose(GObject* object) {
   MyApplication* self = MY_APPLICATION(object);
   g_clear_pointer(&self->dart_entrypoint_arguments, g_strfreev);
+  g_clear_object(&self->engine);
   G_OBJECT_CLASS(my_application_parent_class)->dispose(object);
 }

[cbracken]

Looks great!