[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/SourceFiles/data/data_search_controller.cpp

@@ -53,6 +53,8 @@ MTPMessagesFilter PrepareSearchFilter(Storage::SharedMediaType type) {
 		return MTP_inputMessagesFilterChatPhotos();
 	case Type::Pinned:
 		return MTP_inputMessagesFilterPinned();
+	case Type::Poll:
+		return MTP_inputMessagesFilterPoll();
 	}
 	return MTP_inputMessagesFilterEmpty();
 }