[poll-shared] Added Polls section.

- Introduced SharedMediaType::Poll and registered poll messages
as shared media in data_media_types.

- Added Data::PollMessagesViewer — a producer that loaded poll
message slices for a given history/topic/monoforum peer.
(data_poll_messages.cpp/h)

- Implemented Info::Polls::ListWidget and ListMemento — a new content
widget that rendered poll messages using HistoryView::ListWidget
with a custom chat theme, similar to existing shared media sections.

- Connected the new section into info_memento, info_controller,
info_media_buttons, and info_media_widget so it appeared as a button
in the Shared Media panel.

Telegram/CMakeLists.txt

@@ -688,6 +688,8 @@ PRIVATE
     data/data_photo_media.h
     data/data_poll.cpp
     data/data_poll.h
+    data/data_poll_messages.cpp
+    data/data_poll_messages.h
     data/data_premium_limits.cpp
     data/data_premium_limits.h
     data/data_pts_waiter.cpp
@@ -1111,6 +1113,8 @@ PRIVATE
     info/peer_gifts/info_peer_gifts_common.h
     info/peer_gifts/info_peer_gifts_widget.cpp
     info/peer_gifts/info_peer_gifts_widget.h
+    info/polls/info_polls_list_widget.cpp
+    info/polls/info_polls_list_widget.h
     info/polls/info_polls_results_inner_widget.cpp
     info/polls/info_polls_results_inner_widget.h
     info/polls/info_polls_results_widget.cpp