gpui: Fix Global trait (#11187)

This PR restores the `Global` trait's status as a marker trait.

This was the original intent from #7095, when it was added, that had
been lost in #9777.

The purpose of the `Global` trait is to statically convey what types can
and can't be accessed as `Global` state, as well as provide a way of
restricting access to said globals.

For example, in the case of the `ThemeRegistry` we have a private
`GlobalThemeRegistry` that is marked as `Global`:
91b3c24ed3/crates/theme/src/registry.rs (L25-L34)

We're then able to permit reading the `ThemeRegistry` from the
`GlobalThemeRegistry` via a custom getter, while still restricting which
callers are able to mutate the global:
91b3c24ed3/crates/theme/src/registry.rs (L46-L61)

Release Notes:

- N/A

crates/tasks_ui/src/modal.rs

@@ -3,9 +3,9 @@ use std::sync::Arc;
 use crate::{active_item_selection_properties, schedule_resolved_task};
 use fuzzy::{StringMatch, StringMatchCandidate};
 use gpui::{
-    impl_actions, rems, AppContext, DismissEvent, EventEmitter, FocusableView, Global,
-    InteractiveElement, Model, ParentElement, Render, SharedString, Styled, Subscription, View,
-    ViewContext, VisualContext, WeakView,
+    impl_actions, rems, AppContext, DismissEvent, EventEmitter, FocusableView, InteractiveElement,
+    Model, ParentElement, Render, SharedString, Styled, Subscription, View, ViewContext,
+    VisualContext, WeakView,
 };
 use picker::{highlighted_match_with_paths::HighlightedText, Picker, PickerDelegate};
 use project::{Inventory, TaskSourceKind};