feat: added customization options in main widgets (#312)

* polish channelpreview customization options

* polish channelheader customization options

* polish messageinput customization options

* polish threadheader customization options

* polish channellistheader customization options

* show the parent message if no messages in thread

* fix edit message

* fix channelinfo textstyle

* fix review

* fix useravatar

* add ontitle tap to thread header

* add custom message actions

* add borderradius and button builder

* add ontap to messageinput custom send button

* fix bottom sheet

* add doc

* use placeholder image

* add doc

* fix listtile density

* remove subtitle from ChannelListHeaderTheme.copyWith

* add InputDecoration.merge extension

packages/stream_chat_flutter/lib/src/channel_info.dart

@@ -57,7 +57,7 @@ class ChannelInfo extends StatelessWidget {
         style: StreamChatTheme.of(context)
             .channelTheme
             .channelHeaderTheme
-            .lastMessageAt,
+            .subtitle,
       );
     } else {
       final otherMember = members.firstWhere(
@@ -69,10 +69,7 @@ class ChannelInfo extends StatelessWidget {
         if (otherMember.user.online) {
           alternativeWidget = Text(
             'Online',
-            style: StreamChatTheme.of(context)
-                .channelTheme
-                .channelHeaderTheme
-                .lastMessageAt,
+            style: textStyle,
           );
         } else {
           alternativeWidget = Text(