Restore paste on middle-click on linux (#18503)

This is a partial revert of e6c1c51b37, which removed the middle-click
pasting on linux (both x11 & wayland). It also restores the
`middle_click_paste` option behavior which became unexistent.

Release Notes:

- Restore Linux middle-click pasting.

crates/editor/src/element.rs

@@ -636,11 +636,30 @@ impl EditorElement {
             cx.stop_propagation();
         } else if end_selection && pending_nonempty_selections {
             cx.stop_propagation();
-        } else if cfg!(target_os = "linux")
+        } else if cfg!(target_os = "linux") && event.button == MouseButton::Middle {
-            && event.button == MouseButton::Middle
+            if !text_hitbox.is_hovered(cx) || editor.read_only(cx) {
-            && (!text_hitbox.is_hovered(cx) || editor.read_only(cx))
+                return;
-        {
+            }
-            return;
+
+            #[cfg(target_os = "linux")]
+            if EditorSettings::get_global(cx).middle_click_paste {
+                if let Some(text) = cx.read_from_primary().and_then(|item| item.text()) {
+                    let point_for_position =
+                        position_map.point_for_position(text_hitbox.bounds, event.position);
+                    let position = point_for_position.previous_valid;
+
+                    editor.select(
+                        SelectPhase::Begin {
+                            position,
+                            add: false,
+                            click_count: 1,
+                        },
+                        cx,
+                    );
+                    editor.insert(&text, cx);
+                }
+                cx.stop_propagation()
+            }
         }
     }