Add RoomTimelineStyle that describes a room timeline style used to customize timeline appearance.

Riot/Modules/Room/Views/BubbleCells/Styles/RoomTimelineStyle.swift

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+// 
+// Copyright 2021 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+import Foundation
+import MatrixSDK
+
+/// RoomTimelineStyle describes a room timeline style used to customize timeline appearance
+@objc
+protocol RoomTimelineStyle {
+    
+    // MARK: - Properties
+    
+    /// Style identifier
+    var identifier: RoomTimelineStyleIdentifier { get }
+    
+    /// Update layout if needed for cells provided by the cell provider
+    var cellLayoutUpdater: RoomCellLayoutUpdaterProtocol? { get }
+    
+    /// Register and provide timeline cells
+    var cellProvider: RoomTimelineCellProviderProtocol { get }
+    
+    // MARK: - Methods
+    
+    /// Indicate to merge or not event in timeline
+    func canAddEvent(_ event: MXEvent, and roomState: MXRoomState, to cellData: MXKRoomBubbleCellData) -> Bool
+}