Room screen: timeline message bubbles (#91)

* #34 Create `TimelineItemStylerView`

* #34 Add styler view into different type of cells

* #34 Use placeholder avatar image on room list

* #34 Add `isOutgoing` param to event based timeline item

* #34 Compute `isOutgoing` for timeline items

* #34 Update sender info view in timeline

* #34 Update mock timeline items

* #34 Rename `EventBasedTimelineView` to `EventBasedTimelineSenderView`

* #34 Change padding on timeline cells

* #34 Create `TimelineItemStylerView` to move content into a bubble if needed

* #34 Use styler view in all of the timeline item views

* #34 Make timestamp more readable on images

* #34 Little layout tweaks

* #34 Add changelog

* #34 Fix code smells

* #34 Set text colors on timeline items

* #34 Fix background color of the timeline

* #34 Fix PR remarks

* #34 Set background colors explicitly on remaining screens

* #34 Reduce min bubble width and make it a scaled metric

* #34 Refactor `PlaceholderAvatarImage` to accept a text only

* #34 Fix code smell

* #34 Fix further comments

ElementX/Sources/Screens/RoomScreen/View/RoomScreen.swift

@@ -34,6 +34,7 @@ struct RoomScreen: View {
                 RoomHeaderView(context: context)
             }
         }
+        .background(Color.element.background, ignoresSafeAreaEdges: .all)
     }
     
     private func sendMessage() {