Support adding a caption to media uploads. (#3531)

* Add a composer to the MediaUploadPreviewScreen.

And send it's content to the media upload's caption.

* Use the new compound SendButton (updating relative padding in the toolbar).

* Update snapshots.

* Add unit tests for sending a caption.

ElementX/Sources/Mocks/TimelineProxyMock.swift

@@ -0,0 +1,35 @@
+//
+// Copyright 2024 New Vector Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only
+// Please see LICENSE in the repository root for full details.
+//
+
+import Combine
+import Foundation
+
+extension TimelineProxyMock {
+    struct Configuration {
+        var isAutoUpdating = false
+        var timelineStartReached = false
+    }
+    
+    @MainActor
+    convenience init(_ configuration: Configuration) {
+        self.init()
+        
+        sendMessageEventContentReturnValue = .success(())
+        paginateBackwardsRequestSizeReturnValue = .success(())
+        paginateForwardsRequestSizeReturnValue = .success(())
+        sendReadReceiptForTypeReturnValue = .success(())
+        
+        if configuration.isAutoUpdating {
+            underlyingTimelineProvider = AutoUpdatingRoomTimelineProviderMock()
+        } else {
+            let timelineProvider = RoomTimelineProviderMock()
+            timelineProvider.paginationState = .init(backward: configuration.timelineStartReached ? .timelineEndReached : .idle, forward: .timelineEndReached)
+            timelineProvider.underlyingMembershipChangePublisher = PassthroughSubject().eraseToAnyPublisher()
+            underlyingTimelineProvider = timelineProvider
+        }
+    }
+}