Unify the task source and task canceller API

I moved away from the `Window` struct all the logic to handle task
sources, into a new struct called `TaskManager`. In a happy world, I'd
be able to just have there two functions, of the types:

```rust
fn task_source<T: TaskSource>(&self, name: TaskSourceName) -> Box<T>
fn task_source_with_canceller<T: TaskSource>(&self, name: TaskSourceName)
  -> (Box<T>, TaskSourceCanceller)
```

And not so much duplicated code. However, because TaskSource can't be a
trait object (because it has generic type parameters), that's not
possible. Instead, I decided to reduce duplicated logic through macros.

For reasons[1], I have to pass both the name of the function with
canceller and the name of the function without, as I'm not able to
concatenate them in the macro itself. I could probably use
`concat_idents` to create both types already defined and reduce the
amount of arguments by one, but that macro is nightly only. At the same
time, not being able to declare macros inside `impl` forces me to pass
`self` as an argument.

All this makes this solution more verbose than it would be ideally. It
does reduce duplication, but it doesn't reduce the size of the file.

[1](https://github.com/rust-lang/rust/issues/29599)

1 files changed, 10 insertions, 7 deletions

diff --git a/components/script/dom/textcontrol.rs b/components/script/dom/textcontrol.rs
index 3ed20a2901a..1f276b94ba8 100644
--- a/components/script/dom/textcontrol.rs
+++ b/components/script/dom/textcontrol.rs
@@ -297,13 +297,16 @@ impl<'a, E: TextControlElement> TextControlSelection<'a, E> {
         // Step 6
         if textinput.selection_state() != original_selection_state {
             let window = window_from_node(self.element);
-            window.user_interaction_task_source().queue_event(
-                &self.element.upcast::<EventTarget>(),
-                atom!("select"),
-                EventBubbles::Bubbles,
-                EventCancelable::NotCancelable,
-                &window,
-            );
+            window
+                .task_manager()
+                .user_interaction_task_source()
+                .queue_event(
+                    &self.element.upcast::<EventTarget>(),
+                    atom!("select"),
+                    EventBubbles::Bubbles,
+                    EventCancelable::NotCancelable,
+                    &window,
+                );
         }
 
         self.element