Remove all Backend "get_any_client"-using methods

thumbnail() and download() were remaining.
Use a direct and carefully chosen MatrixClient's methods instead to
avoid problems mentioned in the previous commit 7502c1.

src/gui/Pages/Chat/RoomPane/MemberView/MemberDelegate.qml

@@ -23,6 +23,7 @@ HTile {
 
         HUserAvatar {
             id: avatar
+            clientUserId: chat.userId
             userId: model.id
             displayName: model.display_name
             mxc: model.avatar_url

src/gui/Pages/Chat/RoomPane/MemberView/MemberProfile.qml

@@ -61,6 +61,7 @@ HListView {
             root.currentIndex = 0
 
         HUserAvatar {
+            clientUserId: chat.userId
             userId: member.id
             displayName: member.display_name
             mxc: member.avatar_url

src/gui/Pages/Chat/RoomPane/SettingsView.qml

@@ -82,6 +82,7 @@ HFlickableColumnPage {
 
     HRoomAvatar {
         id: avatar
+        clientUserId: chat.userId
         roomId: chat.roomId
         displayName: nameField.item.text || chat.roomInfo.display_name
         mxc: chat.roomInfo.avatar_url