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/attachment/file_attachment.dart

@@ -104,10 +104,10 @@ class FileAttachment extends AttachmentWidget {
               return getFileTypeImage(attachment.extraData['other']);
             },
             placeholder: (_, __) {
-              return Container(
-                width: size?.width,
-                height: size?.height,
-                child: Image.memory(kTransparentImage),
+              return Image.asset(
+                'images/placeholder.png',
+                fit: BoxFit.cover,
+                package: 'stream_chat_flutter',
               );
             },
           ),

packages/stream_chat_flutter/lib/src/attachment/giphy_attachment.dart

@@ -348,10 +348,10 @@ class GiphyAttachment extends AttachmentWidget {
               height: size?.height,
               width: size?.width,
               placeholder: (_, __) {
-                return Container(
-                  width: size?.width,
-                  height: size?.height,
-                  child: Image.memory(kTransparentImage),
+                return Image.asset(
+                  'images/placeholder.png',
+                  fit: BoxFit.cover,
+                  package: 'stream_chat_flutter',
                 );
               },
               imageUrl: imageUrl,

packages/stream_chat_flutter/lib/src/attachment/image_attachment.dart

@@ -84,10 +84,10 @@ class ImageAttachment extends AttachmentWidget {
             height: size?.height,
             width: size?.width,
             placeholder: (_, __) {
-              return Container(
-                width: size?.width,
-                height: size?.height,
-                child: Image.memory(kTransparentImage),
+              return Image.asset(
+                'images/placeholder.png',
+                fit: BoxFit.cover,
+                package: 'stream_chat_flutter',
               );
             },
             imageUrl: imageUrl,